To set up a music school requires venue. That means you have to hire/buy a place where you can teach your students at. You should also ensure that you can rent out rooms to other music teachers to teach their students, this will help pay for the overall rent.
I would hope that you intially have a largish student group of your own before you even think of starting out. I would say at least 50 students who would attend weekly would be the bare minimum. And that is probably to break even with you working full time.
With that work load expected to increase you would definatly need a group of teachers willing to work with you. To hire a full time professional teacher and to pay them a salary would be out of the question to start off. You need to have to find teachers who would be willing to find time to tutor at your school. Teachers you trust, respect and personally know would be better rather than meeting new teachers in my opinion.
To attrach students you would have to definatly decide what type of school you want to be. If you work as predominantly as a one on one (or small groups) tuition school you have to ensure that your name is spread for that cause. A tution school is more flexible in my opinion since the general public can also join in.
If you are not a tution school and focus more resembling a univeristy where you can aquire a degree, that is a totally different question. You would have to prove your school as worthy to even make consider it becoming a univeristy, you would need high quality dedicated teachers at your school, and students which consistiently find work in the music profession. Start off as a tuition based school, and you can grow from there.
The biggest worry is the rent of the venue. Also the rent of the instruments. You have to really think where you locate yourself, often you might be able to find areas at schools which you can use after school hours. however this will restrict you with the instruments you have available. Locationg is not so improtant but you cant be in an obscure place. You need instruemnts, so you will have to do the math on rentage or purchasing pianos. Often you can hire top quality yamaha uprights for at around 80$AUS a month if you shop around.
You will have to draw up a contract as well for students to sign when they choose to attend your school. These contracts will have to ensure that you can commit students to x amount of lessons. You cannot afford to have students miss lessons, cancel etc, it will lose you money and give you a lot of worries. This way they pay the amount and if they miss lessons without real reason, they still have to pay for it. Maybe harsh but that is how you have to be if you want to run the business.
I see music schools open up and close down and open up all over the place. The one thing that kills them is the rent. You have to keep your costs down in that department. It is just futile paying 3000 in costs a week(rent/instruments etc) and just breaking even, it is very stressful! Afterall, I do strongly beleive, the longer you can actually survive and keep the business up and not costing you money, the more chance you have of sucessess. Word of mouth is a very powerful selling point, and that grows and becomes stronger the longer you stay around.