Interview, no good clothes, Please help S&B

[RosieSpring]

Hi all.
Lurker on this board, first post.
I have an interview for a dream job. School secretary. I have no appropriate clothes and dont know where to start. I live in leggings and big jumpers.
I'm 5.1 and round.

Congratulations!

All the years I worked in schools my first considerations were can you see down my top? Up my skirt? Does it go in the wash? Do I look smart, clean and professional? It's not really a Me and Em or Hobbs type of workplace but school secretaries are the best people in the world!