I completely understand that changes must be made when it comes to adapting any form of media into film. The problem is they changed so much from the original source material and cut out everything that made the musical great.
The best example of this is the new ending (SO SPOILER WARNING HERE). The film ends on the note of everyone becoming friends, putting aside their differences and Evans party is a huge success. The musical ends with Evan realising who his true friends are and deciding he doesn't need the approval of the popular kids to be happy; embracing that whilst he may not be popular he has people close to him he knows he can rely on. It's a more realistic ending with the themes of labelling throughout the play and teaches you don't need the validation of "popular" people to be happy, whilst the films ending feels like the same recycled message that get's given at the end of most films that tackle this concept.
The characters suffer greatly too from the changes made for the film. For example in the film the character of Patrice is unpopular in school as she is the geeky kid who's always getting under peoples skin with environmental messages and being a snitch. Patrice in the musical is unpopular as she has tried being friends with everyone in the school but they have all betrayed her trust and stabbed her in the back in many different ways, therefore she chooses to keep away from others as it's the best way to avoid getting hurt.
It's these changes that make me wonder what the point of adapting the musical was. We could have gotten a truly unique outlook on high school films and the messages they push but instead we got a watered down "13 the musical" that would be unrecognisable from the original broadway show if all the character names where simply changed.
If you've never seen the original musical then I completely understand why this film would fall into the good to okay pile. But compared to the film that could and should have been made, this is simply a waste of time.