Answer: 225 students would have at least one sibling. 
 
Explanation:
Three out of four is the same as 3/4 meaning that if there are 300 students in your school then 3/4 times 300 will give the number of students with at least one sibling. 
3/4 * 300 = 225 
This means 225 students would have at least one sibling.