Most teachers have 4 or 5 years of college behind them, with additional training required yearly to keep their license. Most have dreamed of being a teacher for major parts of their life, even after completing student teaching. Then, reality hits. And even with their excessively high pay, and 3 months of vacation, about 50% leave the field in their first five years.
The science and math types of course leave because they actually know about science and math and get paid real money by business. The others because the classroom management skills, reading and writing skills, the ability to work with parents who are experts on their little precious bundle (you know him-the one killing cats with fire, crawling under doors in the girls bathroom, and other cute tricks)are valued in the real world. Some leave because they don't like kids, some are released forcibly.
Teachers are tough and courageous people-who else would let themselves be shut in with 20 to 30 children for 6 hours a day, 180 days a year, knowing that their financial wellbeing was based on what the little darlings learned.
I was in business management for years, my wife was a teacher. The union is still needed in many school districts. Many of the reasons that unions were formed still exist-experienced teachers fired because they cost more than the first year teacher, meaningless work rules imposed by the elected officials who have power but not ability, attempts to cut pay and benefit packages.
I see no reason to require any one join the union, provided they negotiate their own pay and benefit package and not share in the union packages or receive union assistance when having problems with management.