What south park episode does a lady have alien babies that go to the elementary school?

The South Park episode you're referring to is titled "A Scause for Applause," which is the 13th episode of Season 16. In this episode, a woman named "Mrs. Garrison" gives birth to alien babies, and they eventually end up attending South Park Elementary. The plot satirizes various societal issues, including the concept of "alien" life and the consequences of extreme political correctness.