As a first grade teacher I can tell you that I ALWAYS have the kids write whatever it is that they're called. That's what they practice, that's what gets written, that's what I use. I have some kids whose nickname isn't even close to their given name and I use the nickname exclusively.

Relax.

My DD wrote her nickname.

For kindergarten only, I asked DS's teacher to have him WRITE his full name but call him by his nickname (two initials). He needed all the practice he could get to improve his fine-motor skills.

My son goes exclusively by his nickname (much shorter than his full name) and that's what he wrote on schoolwork and what everyone calls him. Also, unless it's "official" paperwork (school registration), I use his nickname (soccer team sign-up, etc).

Whatever she prefers.

When you fill out the get acquainted paperwork (usually given to parents at Open House) just make note that she goes by Katie.

Which is taught? Nickname or given name? Are kids required to go by and write their full give first name or can they use a nickname. Example: Mary-Katherine, will she be required to write Mary-Katherine on all work or can she use Katie?