How do drug tests work if you have more than just one type of drug in your system?

I have been prescribed Ritalin 30 mg LR and take it daily, and last night I smoked weed, I had one hit from a bong. This is the second time I've smoked, with the first being 6 months ago. If I happen to be selected for one of my schools random selection drug testing within the next couple of days, will they know what drugs I have been on, or could the prescription be proof enough. I'm not really certain how these tests work.