Cablepro is correct in that you will need to work closely with the fire marshall, as he will be the final authority.
Also, let me preface this by stating that it would be good practice to install the sprinklers under these "lofts", as you are likley referring to a dormitory type facility, and sprinklers are one of the greatest life safety tools available. However, below is some help to exclude the sprinklers (again, which I don't recommend)
Are these sprinklers Listed Residential, or Standard Spray sprinklers. If standard spray, you may be able to argue that heads are not required under the beds if they are less than 48" in width. Also, are these beds "fixed in place?" If not, this may also be used to avoid sprinklers under them.....
From NFPA 13, 2002
8.5.5.3* Obstructions that Prevent Sprinkler Discharge from Reaching the Hazard. Continuous or noncontinuous obstructions that interrupt the water discharge in a horizontal plane more than 18 in. (457 mm) below the sprinkler deflector in a manner to limit the distribution from reaching the protected hazard shall comply with 8.5.5.3.
8.5.5.3.1 Sprinklers shall be installed under fixed obstructions over 4 ft (1.2 m) wide such as ducts, decks, open grate flooring, cutting tables, and overhead doors.
8.5.5.3.2 Sprinklers shall not be required under obstructions that are not fixed in place such as conference tables.
One last option is to bring up:
8.6.5.2.1.4* For light and ordinary hazard occupancies, structural members only shall be considered when applying the requirements of 8.6.5.2.1.3.
If you are talking about a loft area, it is likely light hazard, so the only obstructions that MUST be considered are structural members.