“I read the brochure inviting all Jewish teens to attend which didn’t mention anything about a Jewish mother. Even if 25% were non Jews is that ok?”
Why wouldn’t it be? If they advertised: “only students with Jewish mothers allowed!” they would get almost nobody. Use your common sense. They aren’t mekareving everyone who shows up to a mass event on the spot. They’ll get the answer before they zoom in on individuals who have real potential.