I'm a parent of a Berklee student. The first semester, before you get your classes assigned, you have to take placement tests to figure out what level of instrument you are (1=least experience, 5=most experience). In addition, you get assigned an ensemble level. Classwise, you also test to get placed in a level of theory, ear training and other core music classes; some of these you may be able to do online, but if you are advanced, they will still have you test during the audition week to make sure you get put in a class appropriate for your background. The good thing about some of the placement classes is that you could test into a higher level of core classes than a 1st semester freshmen if your skills are advanced enough, which allows you to start taking classes in your major sooner or allows you to get two majors within your time at Berklee.
In regards to how difficult the classes are ... it depends on your background and what level of work you are willing to put into them. My D's first semester had moments where she thought it was super hard or almost bearable, but the last 2-3 weeks of the semester were an all out sprint with finals, projects to be turned in and other requirements. She's now going into her 5th semester and is resting up for a pretty challenging semester. So while there are moments when she 'has a life' she also had moments where I knew she was really cranking, but her friends/peers were also cranking, so they all didn't 'have a life' together, so it was bearable. Work hard, and you can play harder afterwards.