this just doesnt make sense
if you were getting bad grades because of the teacher, your parents would know because they can check you're grades online
teachers and parents also discuss your grades at parent-teacher conferences that happen every 6 weeks depending on where you live
and if you are getting grades that bad im pretty sure your parents would be pissed, unless their expectation are garbage
EDIT: and even if the story is real, its 100% his fault for being on the forums when he should be studying in school. Before he claims that he never gets on the forums at school, look at this quote from him.

It implies that he gets on the forums at school
Even though it was posted in 2014, it can DEFINITLEY still apply in the present day.