As someone who taught a high school programming class for a year, 25% is high, but not as far off as you'd think.
In general, "boot up time" for most students was usually fast, but the problem is that the edge cases (students who forgot their password, typed the wrong URL, didn't follow instructions properly, had a technical error) required individual attention, and would quickly eat up time. Many of these problems came from sending students to a web site they hadn't used before, or logging into a service they didn't use often.
I don't work for Clever (I'd disclaim it if I did. I currently work for a different educational tech company) but I can definitely see how a single sign-on for everything would be helpful to teachers.