Approximately what is the parallax angle of a star that is 20 light-years away?

The parallax angle is the angle between our planet, the star and our planet six months later.

The distance travelled by the Earth in six months is two times the distance from Earth to Sun = 2A.

You can construct a right triangle with the positions of the Earth and the star, where:

The distance between Earth and the Sun (i.e. A) is one leg, the distance from the Earth to the star is the other leg, and the angle is half the parallax angle, which leads to the equation:

tan(parallax angle / 2) = A/distance => parallax angle = 2 * arctan(A/distance)

The distance between Earth and Sun, A, is 1.49 * 10^8 km

20 ligth years = 20 * 9.461 * 10^12 km = 1.892 * 10^14 km

=> parallax angle = 2*arctan( 1.49 * 10^8 km / 1.892 * 10^14 km) = arctan( 7.88 * 10^ -7) = 2*4.51 * 10^-5 °.= 9.02 * 10^-5 °.

Astronomers use other unit: parsecs.

I do not know if in your context you need to use parsecs. Here I just explained you what does parallax mean and its relation with trigonometry, which I think is the useful thing for high school.