It sounds like your grandson is having a hard time with this transition and balancing all the activities. I know if I had to be shuttled to 5 different places every week/daily (new school, 3 classes and after school program), I'd be stressed out and tired, too.
Perhaps your son and daughter-in-law need to chat first with the teacher and get an idea of what could be causing your grandson to think she doesn't like him. It could be he's just not used to the structure of school and having someone redirecting his behavior/activities all day, like teachers have to do. Perhaps, she is actually singling him out for some reason, and if so, it's important to know why so that they can help their son or reinforce what the teacher is trying to do. Perhaps there is no problem with the teacher and classroom and he's simply over-stimulated during the day, which causes him to be nervous and tense at school. As a teacher, I see many children over-scheduled and booked-up with classes/daycare/sports every day that does not allow the child to simply be a child. Usually these children are exhausted in my class and I often have to redirect their negative behavior. I also notice they do not get their work done, or are slow to finish things, didn't have time for homework and are unable to focus on their school work or group activities. It's frustrating to a teacher to see that in so many children. All of us need time to reflect and just "be"...\
With this new transition to school, not enough sleep, extra activities and after school program could be causing him to be be exhausted and unable to focus and take in the new structure and demands of school. It can cause a happy child to act out in class or not be focused if they are sleepy or anticipating something. Its a hard balance for parents in choosing what to sign their children up for, especially when children are new to the school-experience.
My advice is to first check with the teacher to see if there's any problem, adjust his bedtime to allow for more sleep, and maybe re-think the need for 3 classes in addition to the after school program. The after school program is a necessity for working families, but the classes aren't. Let's face it, we don't usually see many adults putting in a full week of work and then taking 3 classes each week...whether fun or not, it takes its toll on your spirit when you don't have the time to re-group, think, and just relax a bit. Good luck!!