Firstly, let me say that every teacher is individual so it's always best to go with your personal knowledge of them. But as the Wife of a primary school teacher and being friends with more teachers than I can count, I can tell you the most common gifts received and subsequently thrown away...

Letters
This is always a beautiful gift for Teachers to receive. Teaching can be a really thankless job and a lot of the time Teachers have no idea of the impact they have had on your child and family. This is the gift I would recommend above all else and is free.

Mugs
Oh Lord! We have an entire cupboard full of 'Best Teacher' mugs and I know the teachers lounge has hundreds that barely get used because teachers love their own identifiable mugs.
I'll admit, we do keep and use a few but that's mostly because we personally like coffees the size of breakfast bowls. Last year we received some beautiful Royal Doulton mugs that we love but in general the gift of a mug can be risky.

Candles / Skincare
This can be a really tricky one. I would always suggest avoiding these items. They are great if you know your Teacher likes them and what their fragrance preference is but if not, steer clear!

Favourite Sporting Team Gifts
I swear, if we receive another South Sydney Rabbitoh's merch item, I'm going to contact Rusty personally and give him a piece of my mind. 
In all fairness my Husband loves it and (I guess) I support his love of the team and game. These are always his favourite gifts and he remembers every single child who gave him his revered Bunnies gifts even years later.
So if your teacher is an avid sports fan, this is always a safe bet.

DIY Baking
Baking can be a fab gift - you know the ones that come in the jars with all the dry ingredients measured out and you add the wet. Genuinely we love these ones because we have 3 kids and baking is the last thing I want to do. BUT! We have zero idea who's fingers have been unwashed and mixed these items together so most of the time teachers will throw these in the bin unless we have confidence in where and who these have been made by (sorry!) so unless you have a friendship with your teacher, I wouldn't bother. 

Gift Cards
This is often seen as the embarrassing gift and I am here to tell you that it is probably the best gift you could give a Teacher.
Teachers are exhausted! Your child may have 13 weeks of holidays a year but your Teacher likely works through them as well as ridiculously long days because the genuinely care about your child and their education. Giving them a $10 gift card for their local grocery store helps a lot - particularly if they have families!

School Supplies
These never go unnoticed and seem small but help hugely for the following year. The beginning of the school year is typically rough so anything that helps ease this period and functioning is super helpful. Ask your Teacher what they need most for the next year and I am sure they will give you pointers.

Treats
Every single Teacher in the history of Teachers has treat drawers. They may not make it known to anyone else, but I guarantee you it is there. 20+ kids a day (all with individual needs) and the responsibility of each one of those children's educational future resting on their shoulders alone for that year is a lot. So most of them snack and treat themselves. Heck, my Husband has a mini fridge under his desk for his Pepsi Max!
Ask your Teacher what they like for treats and supply them for the next classroom of minds they're moulding.

Personalised Gifts
This is a great one for Teachers! Particularly for pens or supplies they will use at school or home. It's meaningful as well as helpful. 
There are so many options for personalisation now on pretty much anything so narrowing down the list of things your Teacher loves is so important here. Then add something thoughtful and personalised to it. 

I hope this helps your search for the perfect gift and remember that most schools have a policy where the Teacher must report (and hand over) the gift if it is over $50 so keeping it under is always best.

Raya