All the schools are on the quarter system, so you need to hit the ground running so to speak, be very organized and have good time management skills. Quarter classes are around 10 weeks so assume within 3 weeks of starting the class, you will have one midterm exam, followed by another at around 6 weeks followed by a final. Many class grades will be based on only 2-3 exams only, so doing poorly on one could impact your final grade.
As a Biology major, you will have labs along with your lectures. Labs can just be as time consuming as your lecture classes, so make sure do not overload your first few quarters. At Freshman orientation, most academic advisors recommend taking a lighter than normal course load Fall quarter to acclimate to the quarter system.
Also be aware of the so called “weed out” classes such as General Chemistry/Organic Chemistry and Basic Biology. Many professors make these courses rigorous to weed out the academically weak.
When we attended the Parent Orientation at UCD several years, the speaker warned the parents to expect your student to call home upset they received their first C grade or even worse D or F, all part of going to college where your peers are equally competitive academically.
All the schools will have rigorous courses and I would say one is not more rigorous than the others. Biology is not an easy major but rely on your strengths and interests to help you succeed. Make sure you take advantage of all the resources available to you if you find that you are having difficulty in your classes: tutoring, group study groups, professor office hours.
None of these schools would have accepted you if they felt you could not handle the workload. Best of luck.