You need to find a special education advocate. You district's special education department should have a list of advocates to work with. These are people who are not employed by the school district who are very familiar with special education law and will work to ensure your son's special education rights are being upheld. This person will attend IEP meetings with you, etc. This principal is way out of line, particularly about not allowing your son into school without an IEP meeting. If the teachers are not following the modifications for your son, they are breaking the law. IEP's are legal documents. It sounds like this high school is way out of compliance. (I'm a public high school teacher telling you this.)

Now, about the attendance issue. Does your son have attendance problems? If so, the principal is probably okay in insisting that his attendance improve for that class. Kids have to be in school to learn. That's the bottom line. So, if he's not regularly attending school, you have to get that issue under control. (I'm not sure that's a problem; I couldn't quite tell from what you wrote, so forgive me if I'm misunderstanding.)

As far as the requirement the principal laid out, I would ask him to put his requirements in writing and you can insist it be entered into IEP documents. That makes it legal and binding. If he is uncooperative, that's where that advocate would really be a great help.