You could do it without any array storage but you would need some way of reading through the file twice; if you can "rewind" the file, or if not you can close and reopen it. First pass will add up all numbers less than 101 and divide the result by the number of numbers*, which will be the overall mean. Second pass will work a line at a time: read in a student's scores, calculate the mean and compare it with the overall.

*and if you can do it that way in general, then you're safe if the instructor then gives you a bigger file.