my maths teacher like to read "a hundred and twenty-one " as "one twenty-one" to make sure no one is confused. How can i read the following numbers clearly?
eg 8975397972
Just say each digit separately.
eight nine seven five . . . . .

First, numbers are much easier to read if you set them off with commas as I've done below.

3,597 three thousand five hundred ninety seven
76,896 seventy six thousand eight hundred ninety six
997,868 nine hundred ninety seven thousand eight hundred sixty eight
90,667,587 ninety million six hundred sixty seven thousand five hundred eighty seven
842,468,460 eight hundred forty two million four hundred sixty eight thousand four hundred sixty
8,975,397,972 eight billion nine hundred seventy five million three hundred ninety seven thousand nine hundred seventy two

Note that its quite common to read three digit numbers as your math teacher does. Also four digit numbers are often read in groups of two so that the above example might be read "thirty five ninety seven." Larger numbers are read pretty much as I have written but note that some people like to through in "and" in various places. I'll let them give you examples.

An interesting difference between BrE and AmE is that "trillion" is different. In AmE one trillion is 1,000,000,000,000. In BrE one trillion is 1,000,000,000,000,000,000. This can make for some interesting misunderstandings in circles where such numbers are used.
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