Напишете полную форму she isn't at school

She isn't at school. Она не в школе. Она отсутствует в школе.

isn't = is + not.

Время в данном предложении - Present Simple.

По этой же схеме сокращаются отрицания с другими вспомогательными глаголами.

Present Simple/Present Continuous: aren't = are + not;

amn't = am+not;

Present Simple: doesn't = does + not;

don't = do + not;

Past Simple/Past Continuous: wasn't = was + not;

weren't = were + not;

Present Perfect: hasn't = has + not;

haven't = have + not;

Past Perfect: hadn't = had + not;

Future Simple/Future Continuous: won't = will + not;

и другие.

Все они переводятся как "не", "нет".