I am not so sure how your school works, but the majority of schools do a first come first serve for classes. Often the priority is given to seniors, then juniors….freshman.
However nothing precludes you from signing up for the section you want I hope, at my school and most other schools its just a matter of time and putting in effort.
We call it hunting at my school. You find a section you need and u continuously check if you can sign up for it. First find the sections (you can check for multiple if you want), BUT please make sure that they can be inserted into your schedule without time conflict. So make sure the section you want, does not conflict with another class you have. Otherwise that section you want might get taken by another student. In fact people transfer classes to their friends, where one student with priority signs up for a class for a person, and then at a later date (usually at late hours when traffic is low) people transfer classes. Sometimes these transfers are intercepted. I have been in the library when this happened, and people were distraught, but nonetheless, once a class is dropped its anyone’s ball game.
My point is: at this instantaneous moment, the good section you want is not up for grabs. That’s okay !
Lots of things happen!! People add and drop classes ALL the time for a variety of life reasons !
Let me just say that in the beginning I was very new to this. I didn’t understand why people would sit on the computer and literally for 14 hours, every 10 minutes, while they study they would hunt for a particular section. When you analyze it in the long run, its worth the time you are spending a hundred times over. Sometimes, like my Gen Chem II, it wasn’t about getting an easier A it was about getting a well-deserved, well-taught A. There is no reason for example to get the hard professor for Bio II, a class at my school that covers zero material for the MCAT. So naturally tension is high to get the easier professor. So pick and choose your professors wisely. And spending the extra time and consideration will mean a great semester to a miserable one.
I have yet not to get the section I want for my schedule. I spend an enormous amount of time before semester starts. Usually I will email my Advisers, and you want to ask them when the system will drop the students for a particular semester. When grades are posted, prerequisite testing. You can outright ask them when the best time to check is. For example, a good time to especially stay on the hunt is when summer grades are posted. Anytime any term grades are posted. This is when the system, at a majority of schools, will drop students who have sadly failed a prerequisite class. Another time is when tuition is due. Sadly this occurs as well and the students get added back to the same class with a fee penalty but the seat is still up for grabs for someone else. All of these high turn-over events will give you a prime time. This is why we call it hunting at my school. You are essentially biding your time in the sand for something to come by.
So while you should prepare for a little harder semester using the advice above – nothing is over until it’s over. So keep looking. I know it sounds crazy, it took some time getting used to, but spend whatever time you need finding these classes, I learned fairly quickly this made semesters possible. When you apply no one will care if you took professors that had a 1 rating on ratemyprofessor, versus someone who had a genuine 4-5 (granted both classes are not online).
Don’t get discouraged by anything. My school system actually logs you out after 5-10 min of inactivity on the pages. This is to prevent people from sitting on multiple browsers, and just clicking one button to check for a class. You actually have to log back in almost every time. That didn’t stop me from grabbing the good professor for all my prereqs.
edit: something to consider. During high volume traffic others will be looking for a class as well. Over summer I got discouraged I wasnt going to get the biochem class I wanted. The prerequisite was done during summer A, and now in summer B I thought no one is really adding and dropping classes, everyone is trying to break from school. Wrong ! the class was added and dropped many times. So it can seem very discouraging, but if you are at a big state school like mine, or wherever, chances are people are continuously altering and fine-tuning their schedule. So dont get discouraged.
Good luck hunting !