Properly "teaching metric units" would be synonymous with using them consistently in all science/math/etc. classes throughout middle + high school. Doing this would mean that when entering college, students would have developed an intuitive sense for the metric system.
If what the schools do is just have a 1 hour class which can be boiled down to "Ok, 1 meter is 100 cm, 1000 g is 1 kg", then I wouldn't call that 'teaching'.
(Additionally, I think teaching a secondary language is a project of a completely different scope than teaching a unit system)
I don't know about others, but all of my (US-based) science classes used metric exclusively. Almost all math problems dealing with distance used metric as well.
(Funny story: I failed a 9th grade math quiz because I didn't know how many inches were in a foot. We never covered the US customary system! It really didn't come up often.)
That said, I still don't have an intuitive sense for converting miles to kilometers, or Fahrenheit to Celsius, simply because it never came up outside of the classroom.
I beg to differ, when I was in college in JHU the text books consistently made use of feet and other imperial units like lbs. Granted there were metric units mixed in as well, but there there were imperial units mixed in as well, often to the exclusion of metric units.
If what the schools do is just have a 1 hour class which can be boiled down to "Ok, 1 meter is 100 cm, 1000 g is 1 kg", then I wouldn't call that 'teaching'.
(Additionally, I think teaching a secondary language is a project of a completely different scope than teaching a unit system)