When you talk to a therapist for the first time, you can ask what the mandated reporting rules are. You can also ask what criteria he/she would use for a mandatory hospital referral. You should be able to ask this upfront before ever talking to them.
Mandated reported rules are that they have to report if they feel you are a danger to yourself or others or if you are in a dangerous situation and are a minor or a senior.
This can be open to interpretation somewhat. If a therapist feels your parents drug use puts you in a dangerous situation, they must report. So ask for their ground rules upfront. If your parents are not neglecting you, are providing for your basic needs (food, clothing, and shelter), and are not physically, emotionally, or emotionally abusing you, you should be safe from a mandatory reporting situation. You are older and able to take care of a lot of your own needs (you could fix your own dinner for example even if it were a sandwich). However, if you have a two year old sibling in the house that is not being cared for, that could be reinterpreted because a two year old should not be expected to make his own dinner. You need to feel confident talking to the therapist so having this conversation up front puts the rules on the table and you know where you stand. Note that older kids are seldom removed from homes unless it is an extreme situation.