Google Translate says "catulina" translates to "cardboard."
"Cardboard" is a general term for any kind of thick, stiff paper. Boxes and cartons are made of corrugated cardboard. Business cards--the little 5 cm. by 9 cm. cards with your name and business on them--are made of cardboard. "Card stock," "oak tag," "poster board," etc. are all kinds of cardboard of different thicknesses.
"Card stock" is more like a thick, stiff sheet of paper than a card. Printers use "card stock" for the outer jacket of a booklet.
A "collage" is a kind of art made by combining many small pieces of paper, perhaps with pictures on them, and gluing them onto a single sheet to form an artistic design.
It sounds as if the students are to use a fairly thin, light piece of cardboard--"cardstock"--to support the collage, and glue things onto it.