If I went back in time to 1800s vienna art school, how could I change the grades?

You'd be waiting a while, since he didn't apply for admission until 1907.

He lacked the talent and credentials.  Finding a way to get him in would not have changed who he was or what he would become - his exposure to racist political ideologies took place in Vienna, his acceptance to the school wouldn't change that.

A more likely solution would be to intervene in his personal life in 1904 or 1905.  With some gentle nudging and social engineering, ensure that he and a beautiful Jewish girl fall in love and marry.  The mid-teens are a powerful time for the formation of opinions, opinions that are often very hard to shake even decades later.  Couple this with covert medical intervention.  Head off his mother's cancer, allowing her to live a longer life.  Perhaps do so in the guise of a Jewish doctor.  This might make her more sympathetic and accepting of her son marrying a Jewish girl a couple years down the road (she was a devout Catholic).

Let me know if you do. Time is by the moment & fades away, held only in memory. Adolf was born in 1889 & would've gone to an art school in the early 20th century. His father sent him to a trade school, instead. Both Adois & Adolf were bullheaded & neither would follow the others lead. Google Adolf Hitler.

Just an aside, when I was in college (it seems a century ago) my science instructor work at Jet propel in Pasadina. He told us that, yes, indeed, they were working on Time Travel. He said, something like, "don't you remember things that never were? Or things that should have been, but weren't?" They made measurements of light (E=MC2, M=E/MC2). That's all that I remember. 

Changing his grades wouldn't change the fact that his artistic talent was only minor and mediocre. If he'd been smart, he would have taken the advice of a professor who thought he had a future in architecture rather than art.

But he wasn't smart.

In any case, Hitler's problems went much further back than his art school exam results.  He grew up in exactly the sort of lower-middle-class Austrian demographic in which anti-Semitism, among other things, had long been endemic.

If you going to go back to visit Hitler just shoot the SOB in the head and get it over with