From the standpoint of whether or not you "need" starting blocks, you mostly need them to keep the kids from feeling like idiots when they go to a meet and see a set of blocks for the first time. To prevent that, even a single set of blocks would be sufficient, they could then all have a chance in practice to set them up and come out of the blocks a few times.
For learning correct starting technique, 3 point is a good introduction, and 4 point is a decent approximation of a block start. A teammate's feet are really not far off from the feel of pushing off of a starting block, so can be sufficient in practice (doesnt really work with spikes, obviously, for the sake of not spiking your teammate).
See if you can borrow a single set from another school or nearby club, or find an old junky pair somewhere for really cheap.
As far as building a set, I wouldn't bother, having a teammate stand behind them to use their feet as blocks is just as good as anything you can make. And anything you build wont prep them for the apprehension of using a real set for the first time and feeling silly when all the rich kids from other schools know how to use them. It sounds silly, but high school kids sometimes really are intimidated by how to set up blocks for the first time.