Junior is skeptical of white teachers for two reasons. First, he is influenced by outside sources to be skeptical, and second, his experience teaches him to be skeptical. In order to understand why Junior is cynical of white teachers in The Absolutely True Diary of a Part-Time Indian, it is important to understand that Junior is cynical about all white people. This stems from a long standing cultural grudge between white people and Native Americans. He has been taught, consciously and sub-consciously, that his race has been oppressed by white people forever. White people are richer, smarter, given more opportunities in life, and trying to keep the Native Americans in "their place" by sequestering them on reservations and then basically ignoring them. And, generally speaking, while the Spokane Indians hate white people, they also believe white people hate them back:

So what was I doing in racist Reardan, where more than half of every graduating class went to college? Nobody in my family had ever gone near a college (56).

This makes Junior even more skeptical of any white person who actually wants to come to the reservation and teach him and his peers. Socially, he puts Indians on the very bottom of the class ladder, so it is strange to him that any white person would move down a rung to be a teacher on the reservation. This is evidenced in the way he describes the white teachers who must live in reservation housing in order to be teachers:

It was like some kind of prison-work farm for our liberal, white, vegetarian do-gooders and conservative, white missionary saviors (30).

In Junior's mind, the only kind of sensible white person who would want to take on such a demeaning task of teaching Native American kids has to have some sort of alternative motive. 

In addition to this, Junior's own experience teaches him to be skeptical of all his teachers. 

Some of our teachers make us eat birdseed so we'll feel closer to the earth, and other teachers hate birds because they are supposedly minions of the Devil. It is like being taught by Jekyll and Hyde (30).

When you combine Junior's combined school experience with his deep seeded and learned sense of racism, it is no wonder he is skeptical of white teachers.