In most jurisdictions, both the goods (raw materials) and the service (class) are being "sold" to the customer, who is the end user and thus the sale is subject to sales tax. So, when your friend charges for the class, that $100 is subject to all applicable sales taxes for the jurisdiction and all parent jurisdictions (usually city, county and state).
The teacher should not have to pay sales tax when they buy the flowers from the wholesaler; most jurisdictions charge sales tax on end-user purchases only. However, they are required to have some proof of sales tax exemption for the purchase, which normally comes part and parcel with the DBA or other business entity registration paperwork in most cities/states.
Wholesalers deal with non-end-user sales (exempt from sales tax) all the time, but your average Michael's or Hobby Lobby may not be able to deal with this and may have to charge your friend the sales tax at POS. Depending on the jurisdiction, if this happens, your friend may be able to reduce the amount the customer is paying that is subject to sales tax by the pre-tax value of the materials the customer has paid for, which your friend already paid the tax on.