Unless you work for a large employer of 25 or more employees, you do not have a right to leave for children's events at school. If you are employed by a large enough employer, you may have some rights you can exercise to attend school events if you give reasonable notice of the event. See Cal. Labor Code § 230.7.
There are no limits on the amount of work you can be asked to do, or on the employer's right to call you into work on short notice. Your choice is to stay an deal with it or find another job.
As to missing pay, you have remedies. You can sue in small claims court, file a lawsuit in superior court, or make an administrative claim with the Division of Labor Standards Enforcement. You might consider waiting to enforce your rights until you find new employment. If you do, be sure to keep all of your documents and very carefully document all of the time you believe you have been shorted. You have at least three years to make a claim for unpaid wages. There might also be a right to penalties and attorney fees. You should consult with an experienced employment law attorney to determine the best approach for you.
Good luck to you.
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