Well, it really depends, one would need to know the restraunts around them and know how long it would to take to get to them.
Now lets say there is one 10 minutes away from the school, but the serving time is a about 30 minutes, then again it would also depend on how fast the student can eat, assuming it takes about 20-30 minutes, the student would most likely be 10-20 minutes late.
Hope this helped!! (And good luck with getting the food!!) Have a nice day!!