i get that the way dance academy characters behave can be so infuriating sometimes, like you do roll your eyes a little exasperately when christian runs away from smth for the 100th time or when tara is being unreasonable or when grace cuts up tara's costume even tho she was doing better and it feels like everyone's regressing but like,, that's how humans are. growth and mental progress are not linear. christian had so much development in s2 and then sammy's death just completely threw him back and destroyed everything that he had worked for. grace WAS doing better but the stress of final auditions and the hurt of being called crazy by ben (which,, you were 100% leading her on mister) and then seeing him back with tara, the hated ex, having more chemistry than she'll ever have with him, makes her fall back into old patterns.
but even then, they're not back at the very beginning because s1 christian would have heard zach telling him off a few episodes back and would have taken it as a sign that everyone hates him and probably dropped out of the academy alltogether, s3 christian takes his criticism to heart and starts actually working on himself. grace saw tara rehearsing in her firebird costume so she knew she had switched back, she knew she was cutting up the wrong costume but she didn't do it to harm tara, not really, she did it as an outlet for all of these negative feelings that were bubbling up again.
progress is not linear, and that can be frustrating to see in fiction because we're conditioned to see it portrayed as such, but i really like that they didn't do it that way in the show