What is the ruling in Islamic law on striking from work, especially in the field of education, for the purpose of improving social conditions, and does this contradict sincerity to Allah and seeking reward in the afterlife?

There is no contradiction between striking from work and demanding legitimate rights, and sincerity in work. Striking is an exceptional, temporary measure to obtain rights. It may even lead to excellence and sincerity in work if the strikers' demands are met, because an employee who receives their full rights is more proficient and energetic.