Slate published this cartoon by Glenn McCoy.
Clearly the man objecting to the teacher's mixing church and state is drawn to look ugly. After all, why get so upset about a silly story about bells and angels?
But what if one asked the question without the overlay of excessive emotion? Should teachers really be telling students that every time a bell rings an angel gets its wings? I wouldn't want teachers to say that sort of thing. After all, what are students supposed to make of it? Teachers whose students are young enough to think that their teachers mean literally what they say (which is apparently the age of the child in this drawing) should be careful not to mislead their students like this.
This isn't so much about religion; it's about how one thinks about the world. Teachers are supposed to be a source of reliable knowledge. In this case the teacher is confusing his/her students by telling them something that is not intended to be taken at face value.
If the students are understood to be sophisticated enough to understand the difference between literal and figurative speech, then what are we to make of the teacher's statement? At its most innocent, it means something like "think of something nice when a bell rings." I don't see any harm in that. But I would think that a teacher has some obligation to make it clear to his/her students that his/her intent is to be understood figuratively — which in this case the teacher apparently failed to do. The father shouldn't be looking to the ACLU for help, he should be looking to the teacher education system, which failed to educate the teacher adequately.