Hold muh beer and watch this!
So as to not waste the thrust pushing the rocket in random directions (as the video you posted shows.)
We had a "soda bottle water rocket" kit when the kids were little, which included a fin unit, and the rockets would go MUCH higher with the fins than without.
If I can find my snot-sucker, I'll try to do a demonstration with straw rockets...
Seems to me that if you're trying to maximize thoughput (rockets per class period) by having multiple launchers, you're going to have a bottleneck in re-pressurizing the storage tank (unless you launch them all at once.) Those irrigation valves are pretty good at sudden "open", but they're very slow to close again. You might do better with a single pad, overlapping the re-pressurization with the positioning of the next rocket. (although that might negate your safety precautions. :=( )
(I've been a model rocket launches with dual 6-pad racks, and the throughput was not impresive :-( )
Now I know the reason why opening a fresh bottle of my favourite cider results in a fuel dump of half the contents.
Another way to "boost" profits.