The Post Primary Teachers Association is opposing plans to let public tertiary institutions set up charter schools.
The proposal is in education legislation currently being considered by the Education and Science Select Committee.
PPTA president Angela Roberts told the select committee it would put tertiary institutions into competition with secondary schools.
"Tertiary institutions are now going to be incentivised to grab kids because there's money in that, from the secondary sector, rather than what we have already committed to doing, which is building strong relationships with tertiary to have more better successful pathways for our secondary students from secondary into tertiary."
There is no sound reason for letting polytechnics and universities own charter schools, she said.