When schools on tight budgets need to raise money for field trips or extracurricular activities, they used to sell cookies and fudge, or magazines you never wanted.

Then, along comes McDonald's with a twist called Teacher's Nights, in which teachers volunteer to become employees for a night at the fast food chain, and the school gets a cut of the proceeds.

An attractive hook for students, as seen on fliers promoting these events, is to turn the tables on teachers and have them take students' orders.

Amid growing concern about childhood obesity and moves to curtail junk food sales to kids, more than 50 teachers unions, along with health advocates, have pushed back with a campaign to shut down McTeacher's Nights.

The most recent voice to join this chorus is the United Teachers Los Angeles. Cecily Myart-Cruz, the NEA Vice President of UTLA, told Earth Eats that the events send a mixed message to children and exploit teachers' roles to sell foods that can harm kids.

"A McTeacher's Night is especially troubling when I, as an educator, am now asking our parents and our students and our communities to come out--seemingly to support the school and field trips--but I'm also asking them to buy this fat-laden, obesity-ridden type of food."

Since January 2013, Corporate Accountability International and Campaign for a Commercial-Free Childhood has tracked more than 120 McTeacher's Nights in the greater Los Angeles metro area.

Nationwide, advocates have tracked more than 600 McTeacher's Night events in more than 30 states.

McDonald's says these events have raised millions for education, with an estimated $2.5 million raised between January 2013 and fall of 2015. But advocates say schools generally receive only 15 to 20 percent of the proceeds and found in a survey that schools receive an average of $1.18 per student.

Read More:

• Teachers Unions Continue to Protest ‘McTeacher Night' (Education World)
• McDonald's Asked Teachers to Serve Fries for Free. Now the Teachers Are Fighting Back (Mother Jones)