Join any space agriculture webinars you see advertised online! These may inform you about opportunities, or may help you get connected with people that have similar interests. You can find these webinars advertised on social media.
If you become an ASGSR member, the ASGSR student slack group has an opportunities channel that is frequently updated. Keep up to date with this group and with other professional groups you’ve joined.
Check out Magnitude.io for opportunities getting your students involved in ground studies for real astrobotany research.
Stay up to date with the Growing Beyond Earth educational program. They are looking to expand the reach of their program, so even if there are no opportunities for you right now, new opportunities with GBE might open up in the future.