Jat can lead all varna roles. When Jat is discussing epics he is no less than a Brahman. That is why - unpadh jat padha barabar padha jat khuda barabar.
Jat when fighting is of highest quality warrior that is kshatriya which needs no explanation as history is full of examples. The Jat people were designated by British officials as a "Martial Race"
Jat when comes out of the war field and takes his hal and jeli he becomes agriculturist of highest quality which entire world knows. As per Mahabharata in this case he is a vaishya caste because farming is considered to be a business as per Kautilya - uttam kheti madhyam baan kare chakari bhukh nidan
But by no means one can call Jat Shudra. Brahmans have called them so because they never obeyed brahmanical dominance.
Jats had their own system of life and culture.
Qanungo has rightly said that Jats do not fit in varna system.