Well, I do hope that you’re not simply using your school rosaries because they are free.
St. Benedict rosaries appear to be the typical, 10 prayers, 5 decat rosaries.
For a 15 decat rosary, you’d need 3 of the St. Benedict ones. You’d have the metal and hail mary’s left from 2. For the 7, you’d need 2 and you’d have 3 remaining decates as well as the metal and the Hail Mary strand of one.
One thing that I must caution you is that rosary makers choose materials based on how they work together. For instance, one may select metal that would tangle if long–but is ok in a short rosary. Looking simply at google images of 5 decat rosaries versus 15, one can observe they are made out of different materials. With the 15 typically being much ‘bulkier’ and therefore tangle resistant.
So, in that respect, I’d caution you to try the 7 one first before embarking on the 15.
Also, one of the things I most notice about Franciscan Rosaries is that they are overwhelming simple and not grand. They are often wooden beads strung on a thick cord.
I’d ask, respectfully, that you ponder this. It is no accident that these rosaries are simpler. Franciscan theology is very much about poverty for God’s sake. While it is possible to make an expensive-looking metal & glass bead rosary, it’s a bit contradictory to the message of the Franciscan way.