We have also had a lot of practice these first two weeks of school with fractions. Students don't even know where to begin with fractions, and by taking away the calculator they are forced to deal with them, forced to think about how they interact with numbers, and forced to finally learn fractions. One of the first homework assignments is a review of solving equations with one problem containing a fraction of a third and I'm always surprised at how many students change this to 0.33. I usually give an analogy the next day like if you have a dollar and have to split it with two of your friends, everyone gets 33 cents and a penny is left over. No biggie. And if you have $100 everyone gets $33 and only a dollar is left over. No biggie. But when you're rich a successful and you and your two business partners have to split $1 billion.......call me up, I'll take your leftovers. It's a dumb analogy but the students like it and I get a lot of students to stop changing fractions to decimals like that.
I do have a lot of motivation for getting the students to do more mental math since I teach a section of AP Calculus (more than half the AP test at the end of the year is no calculator) and most of my Precalc students will take AP Calculus the next year. It's only my second year at this school but historically our students have done the worst on the no calculator multiple choice part of the AP test. Maybe it's because the middle schools at one point stopped teaching long division, or other basic fundamentals that are going away that probably shouldn't. Anyhow, I'm trying to work on my students number sense and doing so by taking away their calculators.