You're not wrong. It should be 'are'. Sometimes, colloquially, we might say 'There's no classes....'. The singular contraction 'There's' is just about acceptable in informal spoken English. It's not correct grammatically, but it's a non-standard form which native speakers often use. I use this form myself sometimes, simply because other people do and because it's easier to say. However, it shouldn't appear as an answer in a grammar book. I wonder if it's a misprint.