(1) On average, one main entrance passes through X students and one side entrance passes through Y students every minute.

2(X+2Y)=560

4(X+Y)=800

Solution: X= 120 Y=80.

(2) The maximum number of students in the school may be 45*8*4= 1440.

The number of people who can pass in 5 minutes is (120+80)*2*5=2000.

The actual number of people who passed is 2000 * (1-20%) =1600.

1600 > 1440

Therefore, the four doors built meet the safety requirements.