What does sole custody mean?

Are you sure that sole custody is what you mean?  Because it is my understanding that that term does not exist in the Colorado system.  Colorado does not have joint custody or sole custody. Colorado uses the term parental responsibility – which can either be joint or primary.  If you equally share in overnight visitation with the minor child, you have joint parental responsibility.  If a parent has less then 90 overnight visitations with the minor child, the other parent is considered to have primary parental responsibility.

Colorado also divides residential responsibility from decision-making responsibility. Sole decision-making responsibility occurs when a parent is able to make all major decisions (education, religious, extracurricular, and medical) regarding the minor child without consulting with the other parent.  Joint decision-making responsibility occurs when the parents have to share the responsibility of those decisions.  And really what wrong with her showing up at the school?  Was she trying to take her out without authority?  I think that you should speak with someone about the limits your ex wife has or that you think that she has.  Good luck.