Laura Herrera’s salary has barely gone up in her 20 years of teaching — about $700 in all.The San Antonio-area teacher takes home about $3,700 a month. About $1,400 goes to rent, and the rest is sometimes barely enough to pay the bills and stretch through the month. There have been times when she hasn’t been able to afford buying insulin to treat her diabetes.The raises she’s received — most of them in the last seven years — barely accounted for inflation.“I’m living paycheck to paycheck,” Herrera said. “If something happens to my car, or I have to take care ...