Our instructional assistants in a small school get almost 5 weeks off when the school closes during breaks, holidays... The beginning salary is just under the $455 per week minimum. Are the employees still nonexempt although they technically only work 47 weeks per year and get paid for all 52?

Not only must they make 455/wk to be exempt, they must also be paid consistent amounts per pay period over the year (salary). So the answer is yes, your employees are non-exempt. Paying them for 47 pay periods in order to make more than 455 won't make them exempt.