This is a simple Excel worksheet, that randomly accesses lists of thousands of words. Just refresh the sheet (change the grade level, or press F9 on a PC) to get a new worksheet that can be printed an used to quiz your students.
The word lists get progressively harder through the grades. My suggestion is to make this a once a day activity at home or in the class - randomly generate a list of 12 words, print it (or read from the computer) and see how many your students can spell correctly.