My take. There are lots of other ways.

I would just say he has a high school diploma in mechanical engineering

Some colleges and polytechnics also offer diplomas. Not surte what kind of school this is.

http://www.worldwideacademics.com/fields/engineering/mechani...

Examen Gymnasium, 4-årig Maskinteknisk Linje > High School ...
www.proz.com › ... › Certificates, Diplomas, Licenses, CVs
5 Apr 2012 – (KudoZ) Swedish to English translation of maskinteknisk linje: High School Diploma in Mechanical Engineering (a 4 years program) [CV ...

I would either say:

"Training college/school for mechanical engineers" (Or Mechanical Engineering possibly)

I base this on the UK system of education, which offers education to this age group in what we call a school. From 16 years upwards, we can to to college, so I am sure this would also work.

--------------------------------------------------
Note added at 3 hrs (2012-11-13 14:03:16 GMT)
--------------------------------------------------

As regards a literal translation, basically we cannot just say "mechanics" very easily in UK English without needing to state which specific area of mechanics this is describing, eg. Automotive mechanics, Aircraft mechanics etc.

In the UK system we generally study this area through "mechanical engineering" - (as per my question in the discussion box at 11.44).
See: http://en.wikipedia.org/wiki/Mechanical_engineering

--------------------------------------------------
Note added at 3 hrs (2012-11-13 14:04:07 GMT)
--------------------------------------------------

You could say "Training school for mechanical engineering" (maybe some would prefer that)