School rifles were usually (not always) M1899 carbines that had a rifle stock cut down slightly to be able to have a front barrel band used that had a bayonet lug and the end of the 22 inch carbine barrel, which was thicker than the muzzle end of a Krag rifle, turned down to hold a bayonet ring.

So, the "school rifle" would often, correctly, have one of many carbine rear sights.

But, many bubba-ed carbines were made to be like school rifles which could have been a rifle cut down which would not necessarily be wearing a carbine rear sight.

What is the serial number of your carbine? Is it a Model 1899? All Model 1899 Krags started life as a carbine.