Richard, if I am listening to the sentence that you are questioning, I hear this as the adverb ‘tout’. 

“C’est tout aussi stressant pour...” is a general statement, not a specific statement referring to 'cette semaine' or 'la Rentrée' - that would be ‘elle est…’

As usual, a ‘stand-alone’ adjective following c’est is masculine - hence ‘stressant’.  

Normally adverbs are invariable, and in most circumstances ‘tout’ as an adverb is also invariable - for instance it is also invariable before ‘aussi’ as part of an adverbial expression.

However, there is a situation in which ‘toute’ (or toutes) would have been needed here. If the expression had been specific and in the feminine without ‘aussi’ eg  ‘Elle est toute stressée ..’, or ‘Elles sont toutes stressées .. ’, ‘agreement’ of the adverb ‘tout’ occurs with the feminine adjective starting with a consonant (but not with a vowel or h muet). An unusual exception, even for French !  

Standalone adjectives after c'est are always masculine

Tout/tous/toute/toutes = Everything/all (of them)/whole/completely in French