Hi we are putting on a production of Grease and the stage design is in the hands of the actors, singers...so we are very unfamiliar with these things.  I was wondering if anyone could sugest how we could go about building a rectangular stage that has to be placed on the actual stage.  It has to be at least 10 feet long and 5 feet high and strong enough for about a dozon people to stand on.  Also it would have to be in two peices so it can be moved with ease.  Any suggestions would be greatly appriciated.

I don't want to stymey your endeavours - But to build a structure 5' high & as you suggest, this is outside your fellow students experiance.
Firstly I would suggest forgetting about it!
Your best bet would to hire or scrounge prebuilt rostra to suit.
Unless you have rostra/risers & treads on hand, your teacher or groups orginisation are wooden tops, to expect you to build it.
Any structure which is to support people & in your case dancing & movement as well. Needs to be erected by experianced &/or qualified builders.
Hit them up for a reasonable budget that will afford them insurance coverage & satisfy thier responsibility of duty of care.
Anyway with all that drivvel aside, the last time I did this show & the 'Bueaty School' scene. I designed it with no extensive rostrums & managed to get hold of a dual track harness flying system, So we had 2 of the T birds flying around, simmilar to that in the film. The 2 bods were tracked in/out from OP& PS & we were able to fly up & down as well. The singer & the  beauty school shielas, entered from a set of 5 tread rostra peset upstage of a painted gauze/scim & set behind the scene. When the bueaty scene started there was a transition of the lighting revealing the bueaty school shielas & the flying T birds. Of course there was the usual dry ice effect, rolling in from the Legs DS of the scrim & the smoke/haze vontained US by the gauze. All in all fairly simple, except for the flying bit, which was under the control of the Head Flyman - a licensed rigger.
You don't have to do a lot to tart the scene up, as it is only one song. So K.I.S.S & make it easy on your mob, but effective on the stage deck as it is only an inset cameo.

I agree with Joe. I would keep it really simple for first timers. You can get much of the same effect that you are talking about by concentrating on the light effects and fog without constructing such a complicated structure. When I am designing for  a group that doesn't do a lot of building, I try to keep the design to a unit set. Incorporate some levels and platforms and think outside the box to make the (like Joe said) the "cameo" scenes work. You'll be much happier with the final product if you design within your builder's capabilities. Good luck with the show.