Where do I enter form 1040 for K-12 education expenses?

The only education expenses you could deduct on your federal tax return for K - 12 would be teacher expenses. They are entered in the Deductions and Credits section, then Employment Expenses, then Teacher (educator) expenses.

There is no federal deduction or credit for K-12 tuition.  Some state do allow a deduction on the state return. If yours is one, that question will come  up in the state interview.

If you had a distribution from a ESA or 529 plan, K-2 tuition is a qualified expense for federal.  You enter the tuition in the 1099-Q section of TT.  It does not get entered anywhere on the federal forms, just the TT worksheet, unless some of the distribution is taxable.