I checked with my colleagues who went to school in the US< it is usual in US middle school as well
From: stds-1788@xxxxxxxx [mailto:stds-1788@xxxxxxxx] On Behalf Of John Pryce
> §8.8.9, Example (i): Replace "2½" by "2+½"? I don't know whether "2½"
> is a common notation. However it may be ambiguous.
It's the usual British notation. E.g. Wikipedia:
> Mixed numbers
> A mixed numeral (often called a mixed number, also called a mixed fraction) is the sum of a non-zero integer and a proper fraction. This sum is implied without the use of any visible operator such as "+". For example, in referring to two entire cakes and three quarters of another cake, the whole and fractional parts of the number are written next to each other:
> .