- The kids were told that 2 to the power of 3 is 8.
For general truths, the present tense can be used.
- I was told the sky is blue but I don't believe it.
The condition of the sky remains the same. Therefore, is is used.
Compare that to:
- I was told the sky was blue. [on that day but not today, for example]
was/is depends on what a speaker wishes to say.
WHEREAS: The sentence below is reported speech:
The kids were told that President Joe Biden would visit their school.
It works like this: will becomes would in reported speech:
"Children, President Biden will visit our school". [Actual speech]
The principal told the children that President Biden would visit the school. That is reported speech.