The NCAA, no matter what division, wants the student athlete to graduate, and to do so on time. During the recruiting process, this should be discussed not only with the coach, but with any advisor that the student athlete will report to. I feel that this is seldom overlooked by many.
As a D1 athlete my son was required to take 18 credits fall and spring semester. These days many are encouraged to begin classes in the summer. Also, things are different for programs with 3 or 4 semesters per calender year.
You dont plan your classes on your own. You have an advisor who works closely with you to make sure that you are taking what is required for your major, and to help schedule around the schedule that the coach has made for practice. And of course your spring travel schedule will determine your classes.
If you have a class and you have to miss, permission has to be awarded in advance. My son needed permission to miss classes last fall, even as a senior.
As far as D1 on line classes are available, but usually you may not take them your first year. This may be different for some schools, but I dont think so.
Remember that the school is responsible for you to stay on task. If you need 132 credits to graduate on time, 12 isnt going to cut it, and remember, another semester is more money needed to spend.
I am more D1 orientated so that is the basis formy answer.
One thing that i do know, at sons program and university, if you continually miss class you will lose your opportunity to play any sport.