For this reason, you have two available options:
1. Update verbiage surrounding the school block to let parents know that if their child has not yet entered school that they could enter that in School Name Other, choose this past year in the year drop-down, and then "N/A" or "Not Yet In School" for the grade drop-down. For that to appear in the drop-down, you would want to go to onBoard > Settings > Inquiry > Current Grade and add whichever option you prefer to this table. (You could even go to Core > School > Transfer/Sending Schools to add a value of "N/A"/"Not Yet in School" and direct families to use that).
You could even go to Core > School > Transfer/Sending Schools to add a value of "N/A"/"Not Yet in School" and direct families to use that for the School Name drop-down
2. Create a separate application for the grade levels that would not have a schooling history and remove the block all together for these grades. You could then use a statement block to ask a question as to whether or not the family has placed their child in some form of schooling/childcare. If you do this, just make sure to deselect the respective grades from your current applications so that families are directed to their correct application after saying which grade their child is looking to enter your school.