You should take the opportunity to promote your fete whenever and wherever you can. The school newsletter is the obvious place to start, however other opportunities include emails to the school database and notices on the school website and social media.
Many local newspapers and radio stations – particularly in regional areas and community radio stations – provide free community service announcements for not-for-profit events. Call up and ask if and how they can provide a listing.
Also don’t discount more traditional advertising methods such as flyers and posters at local shops, libraries, community centres and schools and billboards at the front of your school.