To be honest, i get the cheating (don´t condone it though), they had a nice wholesome relationship, then he came (pun intended), that new surge of sensations plus the fact that the guy is rather popular, a far too good of a trap if you ask me, so from the cheating alone, it would be all good, she got tired and failed to do it the proper way (split and then ride).
Where does she fall is in EVERYTHING else, the teacher said it himself, she just needed to talk now and she would be a foolish girl coerced by a guy, easy.
Now, she is an accomplice, she is willingly saying and backing the lies, and even going as far as to present false criminal charges (hence the framing of the teacher "we would have to charge him if this is true") up to this point it was all just hormonal teens being all hormonal and stuff, but now she and the other guy are straight up being criminal
Also... the staff is so proficient and tactful that is actually killing my suspension of disbelief. I´ve seen so many cases IRL where the first thing the school does (mind you with even worse cases) is to cover their asses before anything, then covering the ass of the better student (read money or academic achievement, in that order)