Doing any kind of meditaiton will help you imrpove your general abilities to focus and concentrate, but it will take at the very least days (if you practice every day) and more likely weeks before any noticable change really starts to kick in. What you can definately do is a basic breathing exercise to calm and focus your mind, just try to breathe slowly and deeply from the lower part of your lungs (your stomach will move in and out rather than your chest like normal), do this for about a minute or so, focusing on tryign tomake the 'out' part of the breath as slow and smooth as possilbe. Do this just before you try to focus your intetnion at night and it will calm you mind a bit.

Also in the mean time why not just set a few alarms on your watch/phone throught the day while at school, this will help keep lucid dreaming in your mind, plus if you do a reality check each time your alarm goes it will make you more liekly to have a lucid dream too.