Only one thing to change "No he podido ir a clase de español" (instead of "ir de clase").
As for your question, it will depend on the region. In Chile people would "relajarse".
Comenzó el año de estudios bien, pero a mitad de año se relajó y sus notas bajaron.
He started the year studying properly, but half way through, he became lazy and his grades went down.
In other countries people say "dormirse" (lit. "to fall asleep")
A mitad de año se durmió y sus notas bajaron.
Half way through the year, he became lazy (fell asleep) and his grades went down.
I hope this helps you.