There are a total of 3 cassettes that you will need to collect during 1 playthrough:
22:37 -
Cassette 1 (Level 2, first area; bottom left)
24:36 -
Cassette 2 (Level 2; in the classroom near the top of the screen)
1:03:50 -
Cassette 3 (bottom of the escape hatch; right side of the screen)