If I currently have pending charges will it show up in a criminal background check for an employer ex. School board

Generally a background check will only show convictions. However, some jobs, like education, may do a more thorough check. But, if you don't tell them about them pending charges and it comes out later, they are more likely to fire you.

I did HR for a school district and it was common for us to see an arrest record even when there was no conviction. What do you mean by "employer ex. school board" and what type of crime will you be charged with?

Pending means they have not pressed any charges yet, so you are merely under investigation. So there is no record of this.
If there is a warrant for your arrest, those normally will show up

Did they arrest you or not, it makes a difference.