Personally, I am very excited for the Mister Agreste episode.
To me, Theo is to Adrien as Lila/Chloe is to Marinette, and I am glad Adrien will have a very good reason to dislike/hate Theo outside of being Chat Noir.
Adrien kinda needs to have a true throne in his side, to have some real beef with someone. The thought of him seeing Theo in school and just immediately getting pissed off and treats him in an extremely cold manner with others seeing it is really important to me. I want others to start treating Adrien as less than perfect, that the "perfect boy" image that Gabriel built around him gets shattered.
It would be good for his character development, I feel.
(ALSO! Theo was confirmed to be a teenager between the age range of 16 to 18 in seasons 1-5. With season 6 being a new school year and if he didn't graduate (meaning he could still be in that age range), then there's still a chance for him to be at the school)