If you want to politely decline the invitation without explicitly saying "no," you can respond with a gracious and tactful message. Here's an example of how you could respond:
"Thank you so much for the kind invitation to the high-school reunion. It's heartwarming to know that you remember and consider me for such an event. I truly appreciate the effort you've put into organizing it.
As much as I'd love to attend and catch up with old friends, I currently have a few commitments and projects on my plate that I must prioritize during that time. Unfortunately, it looks like there might be some conflicts preventing me from joining the reunion.
Nevertheless, please extend my warm regards to everyone from our class, and I hope you all have a fantastic time reminiscing and reconnecting. I'd love to hear all about it afterward.
Wishing you a memorable and enjoyable reunion!"
With this response, you are being honest about having other commitments without explicitly declining the invitation. It shows that you value the invitation and the effort put into organizing the event, even though you won't be able to attend.