When DS was approved for his first math acceleration in 2nd grade, his teacher just switched her schedule so that her math slot was lined up with the GT 3rd grade math slot. However, it is inevitable that schedule snafus occur now and again, basically whenever there were special events such as field trips or assemblies or special testing for either grade. That is one reason why you want the accelerating student to be at the top of the class in ability/knowledge as well as have decent common sense and practical skills. He has to behave himself and quietly do his own thing while his classmates are having their lessons and figure out what he missed and have to make up in his own class.