As mentioned above, fire hose filled with something a bit heavier (gravel for example) is quite good, stays still most of the time. Foam probably won't. Fire brigade used to give it away for nothing once it had come to the end of its life, hopefully they still will.
Another marker is shipping ropes, again with a little begging you might get them from the docks when they are at the end of their life. Old tyres make good corner apexes, I've even seen clubs make their own concrete or even iron castings - these have a good weight to them.
Another popular one - more common with on-road tracks on carpet but OK for off-road too - is square drainpipe sections which you can link together with wooden blocks cut to the correct shape. You could attach some ballast to the ends.
As far as track dimensions go, for 1/10th buggy I'd be looking at 2m lane width minimum, ideally closer to 2.4m. Anything less than 2m and the track starts to feel very small and it is difficult to turn the car full circle within the lanes. For a decent variety in the layout, you are looking at 4 lanes as the track width, and a straight as long as you can manage. Anything less than about 15m will start to feel very short for a 1/10th car, even a slow one.
Using a viewing gallery as a rostrum may not work out so well, you can end up with a strange perspective on the obstacles, and it can take a while to get up and down which will cause delays with marshalling and switching the cars on. Low level stage units are great rostrums (rostra?) if the school has them already.
Sounds like a good project, hope you can get it up and running.
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