A Fond du Lac man was sentenced to 13 years in prison Monday for sexually assaulting an underage girl.  Eighteen-year-old Tory Tuinstra was convicted of 2nd-degree sexual assault of a child last December.  As part of a plea agreement, he entered a no contest plea to the sexual assault charges and prosecutors dropped two other counts.  Investigators say Tuinstra and another man, Shane Berg, sexually assaulted a six-year-old girl at an apartment.  Shane Berg was found guilty last month.  The girl’s mother faces sentencing next month for participating.