[Nevadaref]

BITS is correct on both issues, and he provided a good summary, with the exception that he failed to make a distinction between screening a stationary opponent and one who is moving.
Therefore, I'll just post the exact wording of the NFHS rules on screening and you can read them yourself and hopefully gain a better understanding of the concept.
RULE 10 Fouls and Penalties
Section 6 Contact
ART. 3 . . . A player who screens shall not:

a. When he/she is outside the visual field of a stationary opponent, take a position closer than a normal step from the opponent.
b. When he/she assumes a position at the side or in front of a stationary opponent, make contact with that opponent. It the screen is set within the visual field of a stationary opponent, the screener may be as close to the opponent as the screener desires, short of contact.
c. Take a position so close to a moving opponent that this opponent cannot avoid contact by stopping or changing direction. The speed of the player to be screened will determine where the screener may take his/her stationary position. This position will vary and may be one to two normal steps or strides from the opponent.

d. After assuming his/her legal screening position, move to maintain it, unless he/she moves in the same direction and path of the opponent. When both opponents are moving in exactly the same path and same direction, the player behind is responsible if contact is made because the player in front slows up or stops and the player behind overruns his/her opponent.

If the screener violates any of these provisions and contact results, he/she has committed a personal foul.

A player who is screened within his/her visual field is expected to avoid contact by going around the screener. In cases of screens outside the visual field, the opponent may make inadvertent contact with the screener and if the opponent is running rapidly, the contact may be severe. Such a case is to be ruled as incidental contact provided the opponent stops or attempts to stop on contact and moves around the screen, and provided the screener is not displaced if he/she has the ball. A player may not use the arms, hands, hips or shoulders to force his/her way through a screen or to hold the screener and then push the screener aside in order to maintain a guarding position on an opponent.