The majority of dental offices (and businesses in general) use windows computers. Your school will most likely use windows in their clinic for charting ect.
In terms of a computer for personal/classroom use during dental school, you can use whatever you want. You will be reading PDFs/PowerPoints and using the internet. You can use an iPad, tablet, laptop, surface... whatever you want to use. I think the ideal for me during class is an iPad with the Apple pencil, but you can succeed with anything you chose to use.