First, your variable can't start with a numerical digit, only letters and certain special characters. I believe what you want to do is set a global variable and you can do that by using the window object. What i would recommend is creating your own object variable and using that instead. This is how you would do that:
var infoObject = {
classHours: 127,
currentNumber: 3
}
infoObject['pop_up'+infoObject.classHours+'_'+infoObject.currentNumber] = 'variable contents here';
However, there should normally be no reason to have to create variables in this way. What exactly are you trying to accomplish?