I run a games course at an inner city college. Can i use the free version on the class computers or do i need to pay?

We can’t afford to pay for Unity

We have UDK but I really want to switch to Unity.

Subscriptions to student licenses are available through Studica. We also have educational licenses available to accredited educational institutions. Schools can contact us using our contact form for more information.

http://unity3d.com/unity/faq#section-451

According to the FAQ:

the free version of Unity may not be licensed by… an educational, non-profit or government entity with an annual budget of over US$100,000

I’d say you should look at tanoshimi’s link to this page.