Each flight/squadron is different. First; no one is going to open your mail. Don't send packages, wait 6 weeks and have that sent then. Envelopes/letters are private and no one will touch it. However, once it is open, if someone sent you pictures and such, they could be confiscated until after BCT. Again; each squadron is different. Packages they'll hold for you and won't deliver until after BCT. You don't want packages held at that time. Just send them after BCT.
FWIW; the academy encourages letters. LOTS of them. They understand what you're going through. They want you to keep in contact with home. I do have some suggestions if you like.
1. Prior to leaving for BCT buy a lot of post cards. Give them to your parents, friends, girl/boy friend, etc... Whoever might write to you. Bring a couple for yourself. When they write you, have them include a post card in the envelope that is already addressed and a stamp on it. You are not going to have a lot of free time. The time you have, you will be tired. It's convenient to fill out an already ready post card. Fill it out and drop it in the mail. You won't have a lot of time for writing long letters. You can; just that you're going to be tired/busy.
2. Remember that your family/friends are not going to be able to feel what you feel when you feel it. What I mean is; you're going to have plenty of days where you question being there and believes the place sucks. This is natural. And 10 minutes later, you will feel better and be glad again that you are there. DON'T write the post cards when you're in a "Mood". Either wait until you perk up a little or concentrate on the positives of the academy. Your parents and friends will only be getting letters every few days. One depressing/down post card and make for a terrible few days for your parents/friends.
3. Unless your parents/friends have been through BCT, and even then, they can't understand what you're going through. Nor the why. So, don't write things to them that are negative in a way that's going to tempt them to give you advice or opinions. Unless they've been there, their opinions/advice aren't going to be valuable. And they will recognize your discouragement. If you've got the need for advice, talk to others in your flight. That's the whole concept of the military. To work it out TOGETHER as a team. Your parents/friends can give you words of encouragement, but they aren't part of your team.
Best of luck. You'll have lots of fun. Later... Mike....