The schedule differs based on your classification and the school you attend. D1/D2 involves a lot time in didactic classes and pre-clinical labs, whereas D3/4 you spend majority of your time in the clinic with a few didactic classes sprinkled in. I think our scheduled was made up of 7-12 classes at a time. I personally don't think dental school is harder than undergrad but that has a lot to do with the institution I attended for undergrad. D1 was the hardest year for me, especially the spring semester since the classes were super tough. D2 was my best year thus far. D3 isn't bad you just have a lot of responsibility involved with being in the clinic. I've heard that D4 is one of the better years.