Only four.
04 May 1970 (the invasion occurred on 01 May 1970).
At Kent State University they were shot by Guardsmen after rocks/bricks had been thrown at the men.
The Cambodian border was crossed on the 1st of May. The students were shot on the 4th. So they had since the 1st to prepare for it.
Kent State University.
Kent State.
At Kent State, it was believed a soldier fired a round accidentlly which resulted in other soldiers to fire their weapons. I am not familiar with the story at Jackson State.
One of the most infamous demonstrations against the Vietnam War took place at Kent State University, 1970, when National Guard troops fired on Kent State students and protesters and four were killed and eleven were wounded. The event triggered a nationwide student strike that forced hundreds of colleges and universities to close. Two students were killed by police during a demonstration at another college. The Guardsmen at Kent State claimed they had fired in self defense. Some of those killed were students going to class and not even taking part in the demonstration. There is still dispute over the shootings.
Kent state university
Kent State University.
See: Kent State University Shootings
The only deaths that I'm aware of were the shooting at Kent State University. I think that was 3 or 4.
Kent State, in Ohio.