Hi lishalovespenguins and thanks for the question.
There are a couple pathways people can get into becoming trained and certified in ABA. In order to practice, people require a minimum number of hours in both ABA instruction and supervision. These requirements have been set out by the Behavior Analyst Certification Board (BACB). Their website outlines the various requirements - see http://www.bacb.com/). Once these hours have been met, then you can apply to sit for the certification exam. Pass the exam, and you become a Board Certified Behaviour Analyst (BCBA)
Someone with a Masters degree already (or on the way) in a related field (e.g., psychology, education, science) can take additional coursework in ABA through various on-line programs. People can typically finish these additional courses in a year and then do their 1500 hours with a BCBA. The other option is to get your masters in pre-approved behaviour analysis program where the course sequence is already in-line with the BACB requirements. Sometimes this program includes a practicum component where some (if not all) of the supervised hours can be completed.
As for jobs in education, many school districts contract out to BCBAs in their area, looking to assign support in specific schools where they have identified students. Some of these contracts are with the practitioner directly (via their private practice) or with an outside service provider that has a handful of BCBAs/consultants on-staff. Even still, some districts will hire on a BCBA or “behaviour specialist” as a permanent position. I think it varies from district to district depending on how special education funding is allocated. The good news is that the Individuals with Disabilities Education Act (IDEA) specifically states that behaviour support (or intervention) plans must be based on a Functional Behaviour Assessment (FBA) which is our area of expertise. As there is a need to adhere to this law, there is a role for a ABA (and therefore people trained in it) in the school systems.
If you are considering any coursework or a program in ABA, I highly recommend referring to the BACB site as they have a list of all of the approved programs across the globe. There are various workshops and training available in ABA but not all of them are recognized by the BACB.
Hope this information helps you as you look at the various career paths and options you have.