Learning near Zürich is probably a tough ask.

I know of a few schools, one of which I wouldn't recommend to my worst enemy.

There are two schools in Wallensee, which can be reached fairly easily by train from Zurich.

http://www.windsurfing-walensee.ch/

and

http://www.shirocco.ch/

I have no experience of either of these schools, but the location for the first one, Wndsurfing Walensee is quite nice. There is a small restaurant there and it is a nice place to relax and watch the lake go by. The other place Shirocco is further up the same lake and possibly not quite as easy to get to by public transport. I don't know if either do lessons in English but my guess is that they would at least have a reasonable level of English (most people here do).

One disadvantage of learning in Switzerland is the unpredictability of the wind. While you are not looking for strong wind you do need a bit of a breeze to at least move you forward. Here in Switzerland it is possible that it can be totally windstill.

Another option if you are here for a weekend is to go to Silvaplana (next to St. Moritz). During the summer they get a consistent thermal wind and there is a school there. The scenery is stunning, but the water a touch cold. For me it wins hands down for windsurfing in Switzerland.

If you need any more information get in touch.

Best of Luck.