If it's not safe for them to call you your chosen name in front of your parents I would either not come out to them, or stress very strongly that this cannot go back to your parents and to them you have to be Sam and "she". I would be very conscious that they might slip up on accident even if they are fully supportive, my friends who know often have a pause before saying a name or pronouns because they forget who they're talking to and just don't see me as my birthname anymore, even though they have to use it all the time unless we're alone.
You might want to check whether it's possible for them to treat you male there but not to your parents. It's a quite a bit different (different country, different laws, and work rather than school) but my manager was on the phone the other day pointing out that as work is acting in loco parentis until I'm 18 they really should know whether my parents are supportive of my name being changed and which toilets I'll be using. He said that he wasn't going to call them up and demand that they know all about it, but technically he should. This might be something that you'll want to check up on, because if you're being called Reuben, and you can pass classmates might be funny about you using female bathrooms, but the staff might be funny about you using male ones.
Just a couple of things I think you should consider. I'm not trying to put you off doing it, if you can then by all means do it, but I'm in a situation now where work knows, my legal name change is in the post, my appointment with therapists is booked, and yet my parents don't know. It's a mess to be honest and I wouldn't want anyone else getting into it if they can help it. I've been outted as a lesbian by my school in the past to my parents and that wasn't much fun either.