I'm not sure about hands on learning for dental assisting. That may be fairly specialized work. For that, you may be looking at possible internships with some dentists or dental companies. Past that, I would look at taking advantage of your instructors and lab hours. Maybe trying to see how long your lab is open each day and asking if you can ever get some additional time in there.
Now, if we are talking about more general subjects like math and science, I like the Khan Academy. They have all kinds of videos for all kinds of subjects. Mostly grade school and high school, but the subjects would all still be very similar to what you would be seeing and discussing. http://www.khanacademy.org/
I hope this helps, Mandaa.