You can get them by firing one of the teachers from the Headhunter area (Shopping cart). The teachers that you get there have better general stats than the ones that you hire normally. Once you hire one of these special teachers, you can fire them immediately or later, obtaining some silver tokens in return.

 

Once you reach 80 tokens, you can hire the Headhunter to help you find the best teachers. He also reduces their salary by 50%! So you can have much better teachers for less money. 

 

You can hire the services of the Headhunter as many times as you want for 80 silver tokens each time.