Help me to plan a trip Delhi-Shimla-Delhi and Delhi-Kullu Manali-Delhi for 30-40 students trip.?

Travel by A/C Bus. Triple sharing rooms in decent hotels for two nights.

i m giving you list of hotels you can call them and ask the group hotels rates, i think this will b cheap if you are in group:
http://www.travelchacha.com/hotels/manal...
http://www.travelchacha.com/hotels/shiml...

this will help you

There are many hotels along the way. Most hotels would accomodate 3 people, what age are the students? A/c bus is easy to find as well, check out the website indicated below. good luck

Do you have any travel agents where you are? If so, go to them and let them do the itinerary and costing, this will be the easiest way to go about it and no headache for you!

I think travel portals help you for this.They plan yourmuch better..Visit:
http://www.himachalhotels.in

Suggest you look at Himachal Govt Tourism Websites.
Book a whole bus privately from Delhi.
Make a deal of entire trip; say NO extras.
Book govt tourist hotels. Twin sharing rooms accomodate 3 easily.

a Manali tour Will be more good than shimla, U could also plan to go from Chandigarh, from Chandigarh Manali is 310kms and hectic travelling of 12 hrs. and u could easily get train from Delhi to Chandigarh and u could also get Traveller's a/c coach buses from Chandigarh,
In Manali there are lots of hotel for lodging and even for food, I can suggest Chandramukhi Resorts and cottages which is just 1 kms away from the Manali Market(The Mall).
U could get three rooms cottages with a kitchen, dining hall and sitting room, and the room sizes are so big that even 5 persons can easily stay in a single room. the charges per three rooms cottage are in the range of Rs.2000 to Rs.3500 per day in non-season and in season time it may go more than Rs.4000. make a note that there is a lot of bargaining in all the hotels.