The expected value or mean number of 6's rolled for each child will be 30/6 = 5 and as there are 25 students the mean number of 6's rolled for the calss will be 750/6 = 125.

Is there any indication of how the data is spread? I.e if the data is normal and has a standard deviation then you can use a z-score

For example (x<4.5) = (Z< (4.5 - mean)/std)

Then we can find a value in a standard normal distribution table.

I have created a simulation for you below, could be helpful!