First, you activate an engine. After that, you operate it. But much, much more usual than 'activate' is simply 'start' or 'turn on'.. eg I started my car. (' I activated my car' sounds weird.)
Thank you, Clive. One more question, if you don't bother. A 13 year old, for example, student is in the 2nd (or first) grade of junior high and a 16 year-old is in the 2nd of high school? I mean, do you use the the term "junior high" or are they all called high school students beginning from 1st grade to sixth grade? I would be grateful if you could give me an answer!