To restrict students' access to certain times tables, simply follow the below steps:
When a student is logged into DoodleTables, tap the arrow in the top-left of the screen, then tap Settings
Then select Work Programme along the top (you'll need to enter your teacher password/PIN) > Restricted Tables.
Once you tap on this, you'll then be able to tap on each times tables and choose whether you'd like to lock or unlock it.
If you have set a times table to be restricted, this times table will not appear within the 60s Challenge. For example, if the 1 and 2 times tables are restricted, and all others not restricted then the 60 Seconds Challenge wouldn't ask 1x2, but it might ask 1x3 or 3x2.