As a new diver I agree with the above. (I did buy my own gear beforehand but that is bit different.)
If instructor and DMs shrug you off and say it is OK I like the idea that you offer them your malfunctioning gear to dive in and you take theirs for the dive. That should take care of the situation. Stating something like: "As a new to diving I really don't feel comfortable with this but since you say you are it sure would make me feel better if we switch. This way we will both dive with gear that we are comfortable with."
If instructor and DMs shrug you off and say it is OK I like the idea that you offer them your malfunctioning gear to dive in and you take theirs for the dive. That should take care of the situation. Stating something like: "As a new to diving I really don't feel comfortable with this but since you say you are it sure would make me feel better if we switch. This way we will both dive with gear that we are comfortable with."