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.
"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.