I am very anxious about my job right now, because in my class, there is a student who is likely to be ADHD. Alternately "have ADHD."
He can't join classes or sit on the chair for a long time.
Perhaps "He doesn't participate in class and can't sit still his his chair for long."
He sometimes does something dangerous, like throwing a ball at someone or something.
Alternately "He does dangerous things sometimes, like throwing a ball at someone or something.
I am trying to talk to with him about it, but no good idea for him comes to my mind.
Alternately "but I can't think of a way to fix his behavior" or "but I don't know what to say."
He is also very impulsive, so he kinda easily flips out.
The previous classroom teacher also seemed to have a hard time teaching him. Alternately "dealing with him" or "handling him."
She moved to the another school, so I can't talk about him with her. Or "so I can't talk to her about him."
I know it has only been a week after the new semester started, but I'm not confident that I can handle him.
I hope other students in the class will not be influenced badly by him.
Alternately "I hope he will not be a bad influence on the other students in the class."
They have known him since they entered the school, so I think they are okay.
A new? week starts tomorrow.
It will be long...
My wife is a teacher, so I know it is stressful and difficult. Hang in there!
You English is actually very good. Most of my suggestions are to make it sound more native, but you have the correct meaning and grammar for the most part. Let me know if you have questions and keep up the good work!
