Hi Vashtanerada, (it feels a little strange talking to a flesh-eating shadow, but here goes...)
Yes, seven theremins spaced around a reasonable sized auditorium should suffice. I have organised 19 theremins around the perimeter of a music school auditorium and while not perfect it was "good enough for rock and roll". And yes, surrounding the audience with theremins was just amazing! (Not so good for conventional western music, but stunning for experimental stuff.)
IF you still encounter problems there are two things to consider.
Firstly the orientation of the players. Some theremins can affect others at massive distances. The etherwave pro is a known culprit here, as is the volume antenna of the tVox Tour - both of which you are unlikely to be using, considering their scarcity and cost. Partially blocking the signal with the body of the thereminist has been shown to help, but implies turning one's back to at least one other player. One could perhaps go further and construct chicken-wire walls but note that this is pure guess work on my part.
Secondly, theremins interfere with each other not only through the air but also via the ground (or earth for us Brits) wire. Some (I do not know how many) theremins only require grounding for playability purposes, not for safety reasons. I have been told that this is the case for the Moog Etherwave standard and plus, but DO NOT TAKE MY WORD FOR THIS. I will not accept responsibility for frying your students. Get confirmation from a qualified electrical engineer who has examined the instrument. If you are satisfied that this is the case for the instruments you are using, then the connection can be broken by disconnecting the ground wire at the end of a long extension cable furthest from the theremin. The extension cable will provide sufficient grounding for the theremin to operate correctly.
With regard to building your own, you are on solid ground using one of Bob Moog's schematics. Several have been published in electronics magazines, so are absolutely OK to use for educational and not-for-profit purposes. The only catch is that they are aging, so some components may not be available and substitution will be required. This should, hopefully, not be a problem for your electronics professor. The obvious candidate is the first incarnation of the etherwave. Find the full article here (http://www.element-14.com/community/servlet/JiveServlet/previewBody/16870-102-1-52276/Build%20the%20EM%20Theremin.pdf).
Another possibility, and a build with significantly fewer components, is the Moog Melodia, which has a lovely sound and is still used by at least one professional thereminist (Rob Schwimmer - see youtube here (http://www.youtube.com/watch?v=Ou6oj5fAPz8).) Find the forty-nine-year old article here (http://www.theremin.info/-/viewpub/tid/10/pid/49). (And please, if you do choose this one, do share updated schematics etc. - I would love to build one some day, but don't have the skills to modernise it myself.)
Other than that, use the search function on TW with the keyword [i]schematics[/i] to find lots of discussion about what is good and what is not.
The issue with the B3 is, as Jeff noted, the length of the case. Tuning it does not alter the pitch range - that would require internal changes to the instrument - it alters the size of the playing field, and to get a reasonable size pitch field on a B3 you would have to overlap the volume field, which would mean that presence of the volume hand would affect the pitch of the instrument, particularly when trying to play low notes. Specifically the hand which is nearest to the pitch rod has the greatest effect on the pitch - if that is the volume hand then you are effectively blocking yourself from playing lower notes with your pitch hand.