Ok.....loads of queries!!!!!!! will try to answer all as best as i can!
the biographical sketch gives TISS a chance to know who you are.... stuff that you don't write on ur form... its like a CV or a SOP. the data u put in will be used by them in ur interview. They'll question on what u hv written so keep a copy of the form 4 ur reference... Write a little abt urself, ur strengths, u can mention y u want 2 take up the course etcetera. be positive, highlight ur strengths
There was no aptitude test during our times so it was a GD+essay and after a short-list, interviews happen on next working day. Batch Size is very small (30-35). As far as I know, no preference to any stream (Engineering, psychology, work-ex) is given. Every year, there is a very different mixture of students as far as educational background is concerned. The guy to gal ratio is 3:7!

. Essays and GD's can be on just abou anything. Generally, there is a choice between a social topic (AIDS...), Management topic (Strategic Management) and a current affairs topic.
we have summer placements from which many junta get pre-placement offers and in final placements, we have the likes of P&G, HLL, Hewitt, IBM, Infy, ITC et al coming down. TISS PM&IR is known across the coroprate world.
I really don't think any coaching class is required 4 preparation 4 essay/gd... the only thing to be remembered is that time management is very important. Ensure you manage to conclude your essay within the stipulated time. many of my friends wrote very good intro and body of essays but before they concluded, papers got taken away. In gds, I've heard many groups get disqualified as a whole just coz some ppl in them fought / there was chaos. These are the basic rules for any gd/essay. IMHO, I really don't think coaching classes are required to learn these.
Hope i've answered most of ur queries. I'll still stress on going through the information brochure very carefully.
ATB