the whole appeal of this show was when he was still in high school discovering his powers
part of me also wishes they had far more creative freedom with changing the superman universe...like how much cooler would the show be if Clark had actually told Lex his secret instead of essentially driving Lex away and basically turning him evil? There were so many points in the show where Lex really wasn't a bad guy and was actually a great friend, but they treated the situation like Jonathan (and eventually Clark) "knew" Lex could only be evil as per the comics and thus treated him as such, when the show basically had him drive Lex to become evil due to the mistrust and tension in their relationship.
Of course with Lex on Superman's side, it would definitely be lopsided for most of their opponents and it would make it pretty hard to come up with good adversaries and challenges for the show that wouldn't balloon the cost of production out of control.