Is abortion permissible when the family isn`t ready to receive a new baby?

Abortion is forbidden unless it constituted a serious threat against the mother`s life based on the opinion of trustful and God fearing physicians.

Is it permissible to give lessons to mixed students of the seventh grade?

It is impermissible to give private lessons to mixed students since such an act leads to serious consequences for the students in this grade are teenagers.

Is Zakah (obligatory charity) due on the items used in the trading process such as: cars , shops etc..?

No Zakah is due on equipments used in the trading process such as cars and the like.

Is hair extension permissible?

All Perfect Praise is due to Allah, The Lord of The Worlds 

It is impermissible for the man and the woman to apply a human`s hair as extension, but using artificial hair is permissible for the married woman after obtaining the approval of her husband so long as non-Mahrams don`t see her. And Allah Knows Best.