You can take a deep breath, acknowledge you thought of him and miss him, then opt to concentrate/think about your lesson instead. Keeping grief to private moments before and after school may help.
Don't let it worry you Blake, you'll never forget him but it does get easier over time.Both my parents passed over thirty years ago and even now some thing stupid will click and remind me of them and bring a tear to my eyes,so don't worry Blake just have a good cry, it's just part of life and everybody feels it even if they won't admit it.
It is fine to cry, but if you don't want to then deep breaths can help, try to put yourself into the moment you're in rather than your thoughts. Also, it may help to have a distraction aid like a puzzle or a stress ball may help. If teachers understand you're emotional you may be allowed these in class.
If you feel tearful and want to just get away from it all, then try and go for a brief walk if you're allowed or just try to relax and settle your thoughts and facial muscles.
Grieving is a natural process and you should not deny yourself the time and space to have a cry - can be a great release. I still grieve for my mother, father and daughter - it's part of who I am. Be kind to yourself.
OP can I ask: are you a pupil or a teacher? very different answers required. I remember a similar question a few months go and it turned out the OP was a teacher and most of the answers were assuming they were a pupil.
i agree with mamyalynne,she always give good advice.it is very sad but always remember the good memories you have of your dad.explain to your teacher and friends they will understand.be kind to yourself its natural to cry. my thoughts are with you.