[Grammar] not so much stressful

[Ashiuhto]

Is the following sentence acceptable?

My PE class is very interesting and full of fun. It’s not so much stressful, so we students can enjoy ourselves to the full and get relaxed.

 

[bhaisahab]

Is the following sentence acceptable?

My PE class is very interesting and full of fun. It’s not so much stressful, so we students can enjoy ourselves to the full and get relaxed.

Not really, no. "It's not stressful..." "so much" is wrong here. If you want to compare PE with other classes you could say "It's not as stressful as X class..."

 

[Raymott]

Is the following sentence acceptable?

My PE class is very interesting and full of fun. It’s not so much stressful, so we students can enjoy ourselves to the full and get relaxed.

No, you don't use "not so much" in that way. Here's a way you can use it:
"My PE class was not so much stressful as fatiguing." - It was not very stressful, but it was fatiguing.
You could use "not so very stressful" but I'd advise something simpler, such as bhaisahab's suggestion.