Question 1180805
It is biased because not everybody in the population has a defined non-zero probability of being chosen.  If some students can't possibly be in the first 25, their probability is not some non-zero value (even if very small). It is zero.  Better to choose every third (second, fourth, whatever) student.

This is a convenience sample. The population is all the students in the school. The sample is biased. The explanation is above.