LED PWM is interesting on a 89S52.

Has your Prof defined any resolution, and PWM repeat frequencies ?
Does he want 0% and 100% levels as DC ?
What about PDM (Pulse Density Modulation) ? Google CD4089

There are many ways to tackle this, depends on what matters :

In a 89S52 you could fit 4 channels of 256 play-back booleans into the upper 128 bytes of RAM. ( == 4 LEDs).
Opcode here is MOV A,@Ri
You can cover many modulation schemes here and even hide/embed data in the LED modulation, so you can send status info.

Or, you can use CJNE/DJNZ opcodes in multiple ways, to give 2/3/4/n channels of PWM timing.

All systems will have a fast interrupt, maybe 10-40KHz region, that check each time slot for Pin == On or Off.

With careful crafting you might even avoid needing to push/pop anything in your fast interrupt.