DD will be still be in elementary next year but going to a 7th grade math class at a MS nearby. Any tips?

No experience with that particular situation. Hope it goes smoothly.

Some of my family members did this, but not in the recent past. I can say it was successful for those individuals, at that time.

What is the plan for getting him from middle school back to the elementary school? (For that matter, how does he get to the middle school?) Does he get a locker there, or is he going to have to lug everything with him, and is the middle school teacher ok with that? (Some middle schools here have rules about how much you can carry with you to class, saying that you have to use your locker for excess stuff.)

What is the plan for days when the middle school schedule changes for some reason? This seems to happen all the time for assemblies or holiday adjustments or whatever. If the time at middle school doesn't overlap perfectly with elementary school math time, how will he make up any material covered while he is not there? What will he do if he is at elementary school while they are doing math? Does the middle school ever have "parallel" assignments where, say, they are doing the same stuff in math and in science, and how will they handle it if he is in the math class but not the science class?

Are the schools invested in it working. If they feel their responsibility is met by arranging it is then there will be problems and they will be all on you. If they really want your child to succeed those problems will be dealt with in a much more constructive way.

I agree wholeheartedly with this - but seeing how my questions above are answered may give you a better feel for whether they are invested or not.

These are great questions. Thank you. The district will provide transportation back and forth. I didn't even think about the locker and will ask them this question and the others about schedule changes. DD has been missing her accelerated math class about 5-6 times or possibly more each year in elementary due to special events and it hasn't been a problem, but I imagine it would be more difficult to make up material for a higher level class in a different school. While in elementary they never offered to help her make up the missed days, and we didn't ask so we don't really have any precedent for dealing with this. Any other questions we should ask? Any advice about social issues?

I'm thinking if the district will handle transportation, that's a good sign. The scheduling as far as time missed from other classes due to transportation times would be important, too. Our child has SSA (two years, three years this year in math) but it's been in the same building until this year. No social issues as the focus is really "get into class for math, participate in class, leave." Not a lot of social time. He's going to a high school this year and we transport one way, school the other. It's worked fine. He needs to make sure he has his math book, etc. with him. Are the schools on the same calendar?