The bags sell for $5, with an extra $3 to have a bag personalised with initials.
Marketing Director Nikita MacDonald, 17, says the bags are made by removing sleeves and collars from the t-shirt before they are sewn.
The Shirtly business members all agree the Young Enterprise Scheme is a great way to learn about the business world.
Finance Director Monique Radford, 17, says the slogan for the group is "fighting plastic war for farmers".
They want to keep plastic out of the soil farmers use for crops.
The group has also donated bags to Kaponga Foursquare, where they will be used as returnable boomerang bags.
"The customers at Kaponga Foursquare will use the bag and then bring it back once they're done with it," Claudia says.
Student supervisor Fleur Karasawa says Shirtly has received $1100 from awards they have won through the programme.
The group will have its products at the Young Enterprise Trade Fair at Centre City New Plymouth on July 28.
Regional Challenge One Product Validation Award: First
National Challenge 1 Product Validation Award: First
Taranaki Regional Council Sustainable Business Award: First
BDO Business Plan Award: One of five top businesses
Graphix Business Card Award: First
■ To purchase a t-shirt bag, contact the Shirtly business team on shirtly04@gmail.com