One set of extra schoolrooms is enough in the grade school to be able to unlock the tier 2 education facility. Once you gain the high school, you can close down and demolish the grade school; same with the high school once you've placed a community college or university. The game doesn't discriminate the age of sims, just wealth class, so by this time your sims will have progressed to the higher wealth tiers. You should only need one wing of dormitories for your university. Once maximum education and tech buildings have been reached, you should power this building down for later use.