In this context, 一个教室里只有10个学生， 你不去， 老师一抓一个准, just saying" Since there are only 10 students in the classroom, it will be extremely easy for your teacher finding your absence." 抓 is to catch. 准 means accurance. 一抓一个准也就是准确的抓住。

Interesting question:) The translation of this sentence 一抓一个准 varies according to the meanings of different articles. For example, let’s say a bear is catching fish, 熊在抓鱼，它一抓一个准。means that every time the bear flicks out its paw, it can catch the fish. Because "准" means "accurate", it means that the accuracy rate of bear catching fish is 100%. In your sentence, 老师一抓一个准 emphasizes that the teacher can easily find the students who are absent from class. Because there are only ten students, if the teacher wants to know that the person didn't come to class, that is to “catch the fish” , just try once the teacher will 100% find you.

意思是说，老师肯定知道哪个学生没来上课，因为学生太少，只有十个，学生无法蒙混过关

I'm not Chinese, but from the context, i'd translate what you've got as "...one classroom has 10 students, if you don't go, the professor will let off one time," because it's literally "one catch one allow". That's my non-native interpretation, anyway.