If an assumption proves to be false, is the research then meaningless?
The answer is not necessarily, for two reasons:


It may not actually matter. There are many assumptions that get made in research that are probably not true, in retrospect. Some of these are known to be false from the outset (in my field, it's the assumption that people mix randomly). Said assumption may not actually have all that big an impact on the final outcome, either numerically, or often as importantly, in the subjective takeaway from the project (Should I do X or Y? The difference between X is twice as good and X is 2.17 times as good doesn't really change that you should do X).
You learned something - presumably from actually doing the research, but also about the importance of critically evaluating your assumptions. This is a good thing, and at the high school level, probably the most important.