Drop the ーます from the ーます form and add なさい to make this form.
The coloring in ex #1248 is very confusing. The 'し' looks to be part of '早く', which is not the intention, if I understand correctly.
The Japanese translation is quite nice except the tense. 「…。」と、（先生は）指示しました／指示した。
In the English version of the first example, perhaps "just those" instead of "(just) people." Also, I don't know how they do it in schools abroad, but in Japan, the teacher normally says "should stand up" instead of "should sit down."
I guess this is half way between a 'polite command' and a 'fairly forceful request'.
[V-masu base] + nasai
'[The teacher]' isn't explicitly in the Japanese version, but the English might look odd without it. '(just)' wouldn't be natural in the English - the だけ in the Japanese doesn't explicitly tranlate.