Talk to the teacher about this and how to make you more comfortable. When our guys were in three-year-old preschool, we got a verbal update every day from the teacher about what they did/ate/read/sange/etc. that day. If a "surprise" was part of the day, the teacher mentioned it but didn't give any details. This year, our teacher communicates with us via a notebook that is passed back and forth each day, and I'm assuming the same thing will go for surprises. Perhaps your school can come up with a better way to keep you informed so you can have more meaningful discussions with your son about what he did/learned each day...
As for the surprise thing, I do think you're over-reacting a bit (since you asked). It is fine to clarify with him about the difference between secrets and surprises. However, just as I'm assuming that you like to be able to give him special gifts for birthdays and holidays, give him the opportunity to experience those same feelings. I think it's a great thing that your preschool is teaching him to truly enjoy giving to others and to use his hands and creativity to make something that you would appreciate. Too many kids today grow up without ever giving anything to anyone and, if they do, it is probably something that came from a big box store with mom's or dad's money :(