Hi Rebecca.....!!
This happened because you followed “Empty trash securely” procedure. Open finder>finder preferences>advanced tab>uncheck "Empty trash securely".
I am pretty sure, after doing it you will be able to empty up your trash without any hassle. The reason thousands of files are preparing to trash is because when you use "Empty trash securely”, you are not just emptying the trash but you are completely and permanently emptying the trash to the point where it is impossible to retrieve the deleted files even with the help of "Data recovery software". e.g; deletion of extremely confidential files which are not supposed to be seen by others even with the help of data recovery software.
That 18 files which you discovered and as you mentioned are your old photos and assignments must be a pretty big data thats been emptied securely, which is why its showing 26 thousand items to be deleted. I have gone through the same situation before and "Unchecking Empty trash securely solved my problem". Please reply back and let me know the results.
Regards,
Granel.