Here is the math, assuming a random distribution of students:
1250 student/25 students per class=50 classes per year
50 class*4 years=200 classes over four years for a third grade teacher
200 classes/40 students over four years (if the waitlist is 10 for each grade)= 1 in 5 chance (if only 10 for all grades, 1 in 20--200/10)
It is unlikely, but possible--even more possible if students are ability-grouped in her class right now.