This set has cards for every single possible scale combination for the ABRSM piano exams 2017-18, grades 1 to 8.
Simply print out the page(s) for your student's grade and give them the sheets. They can then cut out the cards and place them in a pot/jar/bag. When they practise, they can pick them out at random to ensure they are practising all of their scales. For grades with more scales, they can sort them into different pots as they play through them - confident, needs some more practice, needs a lot more practice. They should then pick from the "needs a lot more practice" pot most frequently, until eventually all of the scales are in the "confident" pot.
Each card tells the student which scale to play and in what manner it should be played (legato, staccato, which hand(s), which inversion etc) and how many octaves should be played. It is just like having the teacher or examiner giving instructions, and it will ensure students are accustomed to playing each scale on demand.
This resource can be used with every single one of your piano students as they progress through the grades.
NB: I'm not affiliated with ABRSM in any way.