Bolting is the term used when a vegetable crop runs to seed. It is triggered either by a cold spell, a hot spell, or changes in day-length (photoperiod). Annual crops will bolt in the first year, biennials in the second year. Some vegetables (lettuce, mizuna, arugula, etc.) become unusable (bitter) once they bolt.
A tell-tale sign that a vegetable has bolted is the formation of a central stalk. Once you see this you know that the vegetative stage is over and the flowering stage has begun.
I recommend allowing one or two plants to bolt (lettuce and cilantro are good choices). This will not only give students an opportunity to view the complete life cycle of a plant, it will also enable the formation of seeds of which we can save for the following season.
Cool!