The Number 9

So Chad was drawing a picture and wanted to draw a number 9 and asked what it looked like. I said "like an upside down 6". He said "oh ya" and drew it.

After I got done cooking, I told him he did very good but that mommy makes her 9's like this and drew it in the corner. He laughed drew his 9 next to mine and said "yours is an upper case 9, mine is lower case". Lol can you tell what he's learning about in school?