If you ask any ISIS fighter or their sympathizers what sect they follow, their answer would be:
Quran and Sunnah, by the teaching of Prophet and the Righteous predecessors (salaf)
Also for those that think that Salafi is different from Wahhabi:
The Salafi movement is often described as being synonymous with Wahhabism, but Salafists consider the term "Wahhabi" derogatory.
It is often reported from various sources, including the German domestic intelligence service, that Salafism is the fastest-growing Islamic movement in the world.
Many groups, including Al Nusra, Alqeda, Caucasus, ISIS (see source for full list of groups that associate with Salafism) are self described as Salafi (a term applied Salafi Jihadism):
However, let me make it clear that not All people who follow the Salaf (most of the Sunni do) believe in extreme interruption of the Quran and Sunnah (including myself). There are many Salafi scholars who forbade suicide in all of its forms (suicide bombing), for more info on the movement, go here:
Aljoulani (ISIS leader in Syria) also says in an interview with Aljazeera
The creed is from the Quran and Sunnah and the sayings of the Salafi Uluma
Source: https://www.youtube.com/watch?v=qN25VOTw98k @ 0:13 seconds
Da'ish (ISIS) are not Salafi and certainly not Khawarij. They are definitely Wahhabi, as the previous answer has very correctly pointed out.
"Salafi" is not a school of thought, rather a movement calling for the return of the four Sunni schools of thought.
The above being clarified, the answer to your question should become more clear. Islamic State comprises of all four schools of thought. Within its territory, it has judges from all four schools of thought, which are not that different concerning major issues.
The evidence can be found through analysis of its propaganda material, which can be found in "Jihodology".