Since you are 16-19yrs old, tailored shirts, pubs and clubs, are not really interesting to you. No weekend = no chatuchak market too.
For 3 days,
First day: Go Grand Palace. Take plenty of pictures for bragging rights that you have been to Bangkok. Same ticket can go Vimanmek Palace for free.
After all the walking, can go for a decent foot massage around 200B.
Eat some Thai food like phad thai or green curry or tom yam
Second day: Go Platinum mall to buy some T-shirts/jeans, etc. Eat the local snacks around the pratunam area.
In the afternoon, can go Yaowarat (Chinatown) for bird nest/shark fins soup if you all fancy and seafood dinner later at night. Buy some pork floss or chinese products for friends and family.
Third day: Can go Terminal 21 for better quality clothes, etc. Good for people at your age because the decor is themed around cities. Food court is good too.
If still have time and money is no issue, can consider Mdm Tussaud, or aquarium at Siam Discovery and Siam Paragon respectively. There are also football clubs shops, e.g. Arsenal in Centralworld, so look out for them.