If you are going to school next fall, then you might want to hurry up and join, get to boot camp and go to MOS school. All this has to be done before you report to your unit if you are just joining. So, if you were to go to basic in June for example, there is no way you would be done with basic, SOI, and MOS school in time to start school. You would just have to put off school for another semester or so.
If it is an artillery unit, you will probably have to chose a job within arty. If you want to be an 0311, then there has to be a infantry battalion at that duty station. So, you can inly chose the MOSs that that unit requires.
If you get deployed mid-term, you get to withdraw from your classes. In most cases you get your money back for tuition. Wen you return, you would pick up where you left off, and yes, you will be with a "younger" class. I say "younger" because they are just further behind than where you would have been if you didnt deploy. But you would be surprised how many people are not younger than you in age. You will even see other military members who are doing the same as you.
If I were you, and you are interested in being an officer, I would look into first going into the reserves, to get your foot in the door and learn whats its like to be enlisted. Then, find a school with NROTC and choose the Marine program. If you dont accept the scholarship, you can continue to drill with your unit and deploy while taking ROTC courses. Once you become a Junior, you put in your package for OCS and if you get accepted, you can the choose if you want a commssion. If you take the NROTC scholarship, at that time your ties to your reserve unit will be severed and you will belong to the NROTC command and after you graduate, you will have a service commitment of usually 4 years, depending on if you took the scholarship for all 4 years or not. From what I understand, going into the ROTC does not guarentee you will recieve a commission. This is not done until your package is approved. It is very competitive and you must have a GPA of 3.0, score a very high 1st class PFT and certain other requirements.
I hope this helps. I just joined the reserves after 5 years active duty and 15 months of being out. I cant go ROTC because I will be too old by the time I graduate to be accepted into the program, but I did my research and its a program I wish I did earlier. Either way, I have no regrets and Im sure it will be the same with you. Good luck.