A lot of money for schools comes from the government. This is free funding, because a school does not need to pay back for it. However, a school must qualify for the funding. School funding is handled at the state level, so each state has its own criteria for which schools will get funding, and how much funding each school will get. Some states have different amounts for public schools and private schools, but all states require a school to be accredited in order to receive funding.
Tuition is money that some schools will have access to for free. The tuition is the money that students or parents of students pay for the student to go to the school. A school's tuition will depend on the school itself, and is often set by a school board. The toss up is that when a school gets tuition, it means that it is a private school, and private schools often do not get as much in government money as public schools do.
Some schools get money for free because they apply for private grants and win them. A grant is an amount of money that does not need to be paid back, and is usually given after an application process. If your school applies for a grant, it may or may not get one, depending on the other schools that apply and on the amount of the grant. Your school will also have to meet the specific criteria of whoever is giving the grant.
Donations and fundraisers are also free money for schools. When companies or businesses make donations, they simply give specific amounts of money to schools. Fundraisers might be put on by your school, by parents, or by community organizations. They host events or contests or sell donated items to raise money for your school.