An estimated 15,000 teachers and their supporters rallied in downtown Los Angeles on Thursday, threatening to strike should union and school district representatives fail to reach an agreement to reduce class sizes, raise teacher pay, and eliminate the existing system for evaluating educators.

“We do not want to strike, but we know that if we have to, to win the schools LA students deserve, we need to be prepared to do that,” said Alex Caputo-Pearl, president of United Teachers Los Angeles (UTLA), at the demonstration outside City Hall.

The massive rally was the latest action in an ongoing standoff between the 35,000-member teachers union and the country’s second-largest school district. An official impasse was declared earlier this month, after seven months and 18 rounds of contract talks.