One of the meanings of the title of Gary Schmidt's novel The Wednesday Wars concerns the protagonist, Holling Hoodhood, having to spend every Wednesday afternoon alone at school with his seventh-grade teacher, Mrs. Baker--who, by the way, hates Holling (according to him). While the novel is about what happens on those Wednesday afternoons, your question refers to why Holling is the only one left in class once a week.
Half of the kids in Holling's class are Jewish, so every Wednesday afternoon they go to Temple Beth-El to attend Hebrew school. The other half of the kids are Catholic, so the leave and go to catechism. Holling is a Presbyterian with nowhere to go, so he has to stay in school alone with Mrs. Baker.
The Catholic students attend Saint Adelbert's church. Every Saturday afternoon, his Catholic friends attend mass at this church
which had gone modern and figured that it dodn't need to wake parishioners up early.
This is quite different than Holling's church, Saint Andrew Presbyterian Church, which holds services early on Sunday morning.
The Catholic kids attend Saint Adelbert's.