I tried to find this myself, and found that the only classes available were done by Quilt shop. There is one on the top floor in Tanglin mall, but did not offer any formal classes for sewing other than quilts.
Somebody mentined a shop on the top floor in Plaza Singapora, that seems to specialize in arts and crafts, but I am never in that end of town, so I never checked it out.
I also tried the community center in Bukit Batok, but they said these type of classes were not offered.
Hope somebody finds a link to it. I will even consider private lessons
Hope this helps,
Paola