Interference or Legal?

Just a quick question for someone who may have umpired or coached. I am coaching a summer baseball team which goes by high school rules. If a runner on first base or second base, it doesnt matter, attempts to steal or take the next base and slides into the base but cannot reach the base because the fielder has a leg blocking the base is it interference or legal. This is before the ball reaches the base, he slides in before the throw reaches the fielder and is blocked, the fielder then catches the ball right after and tags him out. From what I understand the rules to be I would consider this to be interference because the fielder does not have the right to impede the runner from advancing especially before he has the ball. Other people say its just a good play by the fielder, risky because they could easily get hurt but a good play nonetheless. Anyways I was wondering if someone could help clarify this for me. Thanks