General aviation pilot reported a NMAC in the traffic pattern at a non-towered airport requiring evasive action.

Narrative

Twice in the past week flight school pilots at ZZZ have tried to kill me! Today; a student in Aircraft Y crossed from the south of the downwind leg for Runway XX co-altitude; at pattern altitude instead of the proscribed 1500 ft. Which resulted in myself and an aircraft behind requiring evasive action to avoid a collision. Just a few days ago; another Flight School airplane pulled onto the runway while I was on short final; forcing a dangerous go-around. I asked if there was an instructor in the plane; and was not surprised to hear that yes there was!Unfortunately; this is normal at ZZZ; they dispatch up to 9 aircraft in the traffic pattern at one time. When downwind to Runway XY; they fly into the class D regularly; and when on a far-out base leg to Runway XX; over a populated area; they descend well below the FAA minimum altitude of 1000ft.For too long; this Flight School has been a hazard. Their instructors are poorly trained and don't follow basic rules; then train new students to be dangerous!I could go on; but I believe you get the point!