If you are booking in SL class, then bulk booking is enough... for AC class, pls note that there should be 1/3 availability should be there.. for example if you want to book 10 tickets, then 30 tickets should be available in that class, in your case if 180 seats are available in ac class you can book bulk booking...
bulk booking procedure:
Keep 2 xerox copies of your id proof, in one copy pls write all the passengers name, age, sex at the back of the page...
Write...
more... a letter seeking bulk booking with purpose mentioned...
Thats all.. you can visit Chief commercial manager and book... you cant book in normal Counter with CRS sign, since passengers are more than 12..
P.S: this is the procedure in MAS..