Seniors applying to college should write their college application essays in the summer prior to senior year, which will help relieve some the stress during the busy school year. Essay prompts are available on the college application websites, and below. Most supplemental questions for each college (there can be a lot for some) are available now and don't change much from the prior year. The University of California and Common App open August 1 (both are rolling out a new look with updates) and the Coalition app can be filled out anytime beginning in 9th grade. The California State schools don't require an essay or supplemental question.
Common App prompts: https://www.commonapp.org/apply/2019-essay-prompts
UC App prompts: http://admission.universityofcalifornia.edu/how-to-apply/personal-questions/freshman/index.html
Coalition Application prompts: http://www.coalitionforcollegeaccess.org/essays.html