Although these two are really different as Anniversary Anyway focuses more on the pleasing side of the teenager's life and on the other hand The Guilty Secret gives importance to the sides of life of different teens-Happy, Painful, Beautiful, Memorable, Fun, and treasurable moments of life but shows most of betrayal and of secrets.
The thing that has in common is how sweet the development of relationships is in high school. In short, Romance is the thing that makes them similar- just sweet and delightful :)

big friend group in both
multiple couples
high school
multiple leads
love revolution is very lighthearted and fun

It's very similar it because it's also about school and it's was really fun to watch. Even the covers look alike

A 25-minute “music drama” will wrap up the F4 saga in omnibus format, featuring the four F4 members played by Lee Min-ho, Kim Hyun-joong, Kim Bum, and Kim Joon five years after we last left them.

Due to Ku Hye Sun’s accident (Geum Jan Di), KBS broadcasted a Boys Over Flowers special and featured the F4 actors - Lee Min Ho, Kim Hyun Joong, Kim Beom, and Kim Joon - and be hosted by Han Seok Joon and Hyun Young. Viewers were also treated to some deleted scenes from BOF.

Both are quite the same just the time or the era of both the stories Are different. Both have Historical, Romance, Action, Political, and Power Struggle in both the stories.