I feel like creating a metric to judge how well high school shows do at casting age-appropriate actors.

This article already did that, from a few years ago. I liked their average age-gap as the metric, so I'll use that :) https://www.vice.com/en_us/article/evgzvj/inside-the-phenomenon-of-grown-adults-playing-teenagers-on-tv

How:
-Release year counts as the reference year (even if a pilot might have been filmed a lot earlier).
-This is only about first seasons
-Character ages based on what I find, which might be estimates or not super accurate
-Actor age just by year, exact dates are irrelevant

And i'm learning about U.S. school ages :)
14 to 15 years old: Freshman > 14.5
15 to 16 years old: Sophomore > 15.6
16 to 17 years old: Junior > 16.5
17 to 18 years old: Senior > 17.5
Going by these half-year numbers, unless I find other info.

Here we go.
In no particular order:

Dawson's Creek (1998)
Average age gap: 3.75

Dawson (16) - James Van Der Beek (21)
Joey (16) - Katie Holmes (20)
Pacey (16) - Joshua Jackson (20)
Jen (16) - Michelle Williams (18)

Not bad. It got a lot worse in S2 when they added 27y old Kerr Smith and 30y old Meredith Monroe ;)

Gossip Girl (2007)
Average age gap: 3.79

Serena (16.5) - Blake Lively (20)
Blair (16.5) - Leighton Meester (21)
Dan (16.5) - Penn Badgley (21)
Nate (16.5) - Chace Crawford (22)
Chuck (16.5) - Ed Westwick (20)
Vanessa (16.5) - Jessica Szohr (22)
Jenny (14.5) - Taylor Momsen (14)

Gilmore Girls (2000)
Average age gap: 4.7

Rory (15.5) - Alexis Bledel (19)
Lane (15.5) - Keiko Agena (27) !
Dean (15.5) - Jared Padalecki (18)
Paris (15.5) - Liza Weil (23)
Tristan (15.5) - Chad Michael Murray (19)

Buffy the Vampire Slayer (1997)
Average age gap: 8.5

Buffy (15.5) - Sarah Michelle Gellar (20)
Willow (15.5) - Alyson Hannigan (23)
Cordelia (15.5) - Charisma Carpenter (27)
Xander (15.5) - Nicholas Brendon (26)

Definitely the highest age gap of all the shows so far.

The O.C. (2003)
Average age gap: 6.83

Marissa (15.5) - Mischa Barton (17)
Summer (15.5) - Rachel Bilson (22)
Ryan (15.5) - Ben McKenzie (25)
Seth (15.5) - Adam Brody (24)
Luke (15.5) - Chris Carmack (23)
Anna (15.5) - Samaire Armstrong (23)

Veronica Mars (2004)
Average age gap: 5.5

Veronica (16.5) - Kristen Bell (24)
Wallace (16.5) - Percy Daggs III (22)
Logan (16.5) - Jason Dohring (22)
Duncan (16.5) - Teddy Dunn (24)
Weevil (17.5) - Francis Capra (21)
Lilly (15.5) - Amanda Seyfried (19)

Outer Banks (2020)
Average age gap: 5.67

John B (17) - Chase Stokes (27)
Kiara (17) - Madison Bailey (21)
Pope (17) - Jonathan Daviss (21)
JJ (17) - Rudy Pankow (21)
Sarah (17) - Madelyn Cline (22)
Topper (17) - Austin North (24)

Just guessing these kids are 17ish?

Skins (2007)
Average age gap: 1.72!

Tony (16.5) - Nicholas (18)
Chris (16.5) - Joe (20)
Sid (16.5) - Mike (19)
Cassie (16.5) - Hannah (18)
Michelle (16.5) - April (18)
Jal (16.5) - Larissa (18)
Anwar (16.5) - Dev (17)
Maxxie (16.5) - Mitch (18)
Effy (14.5) - Kaya (15)

My So Called Life (1994)
Average age gap: 3.33

Angela (15.5) - Claire Danes (15)
Rickie (15.5) - Wilson Cruz (21)
Brian (15.5) - Devon Gummersall (16)
Rayanne (15.5) - A. J. Langer (20)
Jordan (17.5) - Jared Leto (23)
Sharon (15.5) - Devon Odessa (20)

Freaks and Geeks (1999)
Average age gap: 2.63

Lindsay (16.5) - Linda C (24)
Daniel (18.5) - James F (21)
Nick (16.5) - Jason S (19)
Ken (16.5) - Seth R (17)
Kim (16.5) - Busy P (20)
Sam (14.5) - John F D (14)
Neal (14.5) - Samm L (17)
Bill (14.5) - Martin S (17)

Friday Night Lights (2006)
Average age gap: 6.88

Smash (16.5) - Gaius C (23)
Matt (15.5) - Zach G (24)
Lyla (16.5) - Minka K (26)
Jason (17.5) - Scott P (27)
Tim (16.5) - Taylor K (25)
Tyra (16.5) - Adrianne P (23)
Landry (14.5) - Jesse P (18)
Julie (14.5) - Aimee T (17)

Saved by the Bell (1989)
Average age gap: 1.17

Zach (14.5) - Mark-Paul Gosselaar (15)
Slater (14.5) - Mario Lopez (16)
Screech (12.5) - Dustin Diamond (12)
Lisa (14.5) - Lark Voorhies (17)
Kelly (14.5) - Tiffani Thiessen (15)
Jessie (14.5) - Elizabeth Berkley (17)

incredible!

Never Have I Ever (2020)
Average age gap: 6.7

Devi (15.5) - Maitreyi Ramakrishnan (19)
Fabiola (15.5) - Lee Rodriguez (21)
Eleanor (15.5) - Ramona Young (22)
Ben (15.5) - Jaren Lewison (21)
Paxton (16.5) - Darren Barnet (29) !

Elite (2018)
Average age gap: 4.6

Marina (17) - María (22)
Samuel (17) - Itzan (21)
Guzman (17) - Miguel (22)
Christian (17) - Miguel (22)
Polo (17) - Álvaro (22)
Ander (17) - Arón (21)
Nadia (17) - Mina (25)
Omar (17) - Omar (20)
Carla (17) - Ester (18)
Lu (17) - Danna (23)

Beverly Hills (1990)
Average age gap: 5.5

Brenda (15.5) - Shannen (19)
Brandon (15.5) - Jason (21)
Kelly (15.5) - Jennie (18)
Steve (15.5) - Ian (26)
Andrea (15.5) - Gabrielle (29) !
Dylan (15.5) - Luke (24)
David (14.5) - Brian (17)
Donna (15.5) - Tori (17)
Scott (14.5) - (16)

Sex Education (2019)
Average age gap: 8.07

Otis (16) - Asa Butterfield (22)
Eric (17) - Ncuti Gatwa (27)
Maeve (16) - Emma Mackey (23)
Adam (17) - Connor Swindells (23)
Jackson (16.5) - Kedar Williams-Stirling (25)
Aimee (16.5) - Aimee Lou Wood (24)
Lily (16.5) - Tanya R (28)

One Tree Hill (2003)
Average age gap: 4.3

Lucas (16.5) - Chad Michael Murray (22)
Nathan (16.5) - James Lafferty (18)
Peyton (16.5) - Hilarie Burton (21)
Haley (16.5) - Bethany Joy Lenz (22)
Brooke (16.5) - Sophia Bush (21)

BEST to WORST age gaps (so far):

1.17 - Saved by the Bell
1.72 - Skins
2.63 - Freaks and Geeks
3.33 - My So Called Life
3.75 - Dawson’s Creek
...
6.70 - Never Have I Ever
6.83 - The O.C.
6.88 - Friday Night Lights
8.07 - Sex Education
8.50 - Buffy the Vampire Slayer

all of this was brought on by High school junior Paxton on Never Have I Ever being played by a 29y old

to be continued ...

Pretty Little Liars (2010)
Average age gap: 6.0

Spencer (16.5) - Troian Bellisario (25)
Hanna (16.5) - Ashley Benson (21)
Aria (16.5) - Lucy Hale (21)
Emily (16.5) - Shay Mitchell (23)
Alison (16.5) - Sasha Pieterse (14) !
Maya (16.5) - Bianca Lawson (31) !

The Vampire Diaries (2009)
Average age gap: 5.67

Elena (16.5) - Nina Dobrev (20)
Bonnie (16.5) - Kat Graham (20)
Caroline (16.5) - Candice Accola (22)
Matt (16.5) - Zach R (24)
Jeremy (14.5) - Steven R. M (21)
Tyler (16.5) - Michael T (24)

excluding the "age-less" vampires ;)

Glee (2009)
Average age gap: 7.5

all sophomore (15.5)

Rachel - Lea (23)
Finn - Cory (27)
Kurt - Chris C (19)
Quinn - Dianna (23)
Puck - Mark (27)
Tina - Jenna (23)
Mercedes - Amber (23)
Artie - Kevin (21)
Santana - Naya (22)
Brittany - Heather (22)

Derry Girls (2018)
Average age gap: 11.6

Erin (16) - Saoirse-Monica Jackson (25)
Orla (16) - Louisa Harland (25)
Clare (16) - Nicola Coughlan (31)
Michelle (16) - Jamie-Lee O'Donnell (31)
James (16) - Dylan Llewellyn (26)

new record

Roswell (1999)
Average age gap: 6.07

Liz (15.5) - Shiri Appleby (21)
Maria (15.5) - Majandra Delfino (18)
Max (15.5) - Jason Behr (26)
Isabel (15.5) - Katherine Heigl (21)
Michael (15.5) - Jason Behr (22)
Alex (15.5) - Colin Hanks (22)
Kyle (15.5) - Nick Wechsler (21)

Dare Me (2019)
Average age gap: 5.5

Addy (17.5) - Herizen F. Guardiola (23)
Beth (17.5) - Marlo Kelly (23)
Tacy (14.5) - Alison Thornton (20)
RiRi (17.5) - Taveeta Szymanowicz (23)

Impulse (2018)
Average age gap: 9.0

Henry (16) - Maddie Hasson (23)
Jenna (16) - Sarah Desjardins (24)
Townes (16) - Daniel Maslany (30)
Clay (16) - Tanner Stine (23)

update on the BEST to WORST high school show age gaps (so far)

1.17 - Saved by the Bell
1.72 - Skins
2.63 - Freaks and Geeks
3.33 - My So Called Life
3.75 - Dawson’s Creek
...
7.50 - Glee
8.07 - Sex Education
8.50 - Buffy the Vampire Slayer
9.00 - Impulse
11.6 - Derry Girls

Euphoria (2019)
Average age gap: 4.75

Rue (17) - Zendaya (23)
Jules (17) - Hunter Schafer (20)
Lexi (17) - Maude Apatow (22)
Maddy (17) - Alexa Demie (25)
Nate (18) - Jacob Elordi (22)
Kat (18) - Barbie Ferreira (23)
Gia (13) - Storm Reid (16)
Cassie (18) - Sydney Sweeney (22)