Conditional patta

Sir,

My grand father bought a land 30 years before in salem, which is conditional patta land(conditional patta given to SC). We are belong to ST comunity.  I want to know whether

1. ST comes under depressed class or not?

2. whether our purchase is valid or not?

3 If not valid whether that land goes to  person who have sold?

4. Now who ever sold to us agin sold to some other person after 30 years, is this valid ?

5. what action i can take this regard?

Please provide your sugestion.

Regards

senthil