Is it acceptable to take some exercises directly from the text book when preparing exercise sheets?
From a practical point of view, taking a few exercises for internal classroom use will probably not cause copyright problems, depending on the local laws. Teaching is one of the fair use possibilities in the US, but I am no expert in law, and I don't know how far it goes.

Assuming US law applies, here are a few rules for fair use, and I believe you fulfil them all: non profit, for educational purposes, on content more factual than artistic, extracting small parts, and no net effect on the market. Some other sources also add restricted access to the student as a point in favour. It is always safer and more ethical to add a reference to the original book.