Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.

 

Lying is not allowed in Islam and as you described it, it is a form of lying.

 

I suggest that you talk to the authorities to make it a rule that for teachers even if they do not teach they are paid their salaries because of the contract regardless of how many days were taught or not.

 

In many fields of jobs this is practiced that you hire someone and they can finish before time and their salary keeps rolling with agreement between two sides.

 

If you took money in that unlawful way, in the future try to donate to the poor upon ability every once a while some money to make up for the income your took unlawfully.

 

Almighty Allah knows best.