There is a way to do this, but it requires a shift in how you think about the cell. The user asking this question wants the impossible in Excel: a single cell that both holds raw input and performs a calculation on that same input. Excel does not allow a formula to overwrite the very data you type into it. That is not a limitation of your skill level; it is a fundamental rule of how spreadsheet logic works. The value you enter is the value that exists, and a formula is a separate instruction that lives in its own space.
So what does this mean for you, the educator? It means you have two practical paths forward, and both solve the real problem you are trying to solve. The first is to use a helper column, which you already know how to do. The second, and more elegant approach, is to use a separate cell for the score and a second cell for the percentage, then format the results so your gradebook still reads cleanly. But if you are determined to type a number and see a percentage appear in that same spot, you are asking for a behavior that no spreadsheet application supports. That is not a failure of your request; it is simply the boundary of the tool.
What you can do instead is reframe the workflow. Keep a column where you type the raw score, like 45. Then, in the adjacent column, write the formula that divides by 53 and formats as a percentage. You can hide the raw score column if you want a clean visual, or you can label both columns clearly. This gives you the exact outcome you want, accurate and automatic, without fighting the tool. The percentage is still calculated instantly, and you still only type the number once. The only difference is that the formula lives in a neighboring cell, not the same one.
The deeper point here is that spreadsheets reward structure over cleverness. The moment you accept that input and calculation are separate actions, you stop wrestling with the software and start using it well. You do not need a workaround for a feature that does not exist. You need a system that works with the tool, not against it. Put the score in one cell, the formula in the next, and you have a gradebook that updates itself every time you type a new number. That is not a compromise. That is the solution.