My son started moving up a grade in math in 1st grade - he is now in 4th, and it's been working beautifully for him. Scheduling was a problem this year - but they worked out that if he attended 5th grade specials as well as 5th grade math, all would be well - so that's what he's done all year
This fall, in 5th, he will go to middle school for math - not quite sure how that wil happen, but the Principals are confident they can work it out
So great to hear you also have a great school to work with !