Any company can give you leave for any reason they deem appropriate. Military and medical are protected by law. In the case of education, it would really depend on the company, their current staffing, and the manager who makes the decision.

Your seniority will generally not accrue while you are on longer-term voluntary leave.

Depending on your timeline, it might be beneficial to take a regional job (one with no training contract) fly the line for a while, then ask for leave. If they say no, just resign (with plenty of notice)...they might change their mind at that point and just give you the leave. If not, then they (or another regional of your choice) would almost certainly hire you back upon your return. This way you are a known quantity with 121 experience when you return from overseas.