First and foremost, I am sorry to hear about this very disturbing situation.
To answer your question, the liability of the school/school board is going to depend upon a great deal of additional factors including knowledge of the offender, where the incident occurred, etc., etc.
You should also note that you have 180 days from the date of incident to file a notice of claim with the governmental entity. Therefore, you should consult with a local attorney as soon as possible.
I agree with Attorney Jerabek's answer. I am only writing separately to emphasize that the law is VERY complicated and you need to consult with an experienced civil attorney who also has experience with law suits involving governmental and quasi-governmental agencies/entities. I hope you daughter is doing well and wish her nothing but the best in recovering from her ordeals.
An important consideration in evaluating whether your Daughter has a potentially viable civil money damages claim/cause of action against the School District is whether there is any evidence tending to show that, before your Daughter was assaulted: 1) The Classmate who sexually assaulted her had previously sexually assaulted another student at school; and 2) The School learned of the first sexual assault but failed to report it to Law Enforcement. Talk with your Daughter and her school friends about any knowledge they may have in those regards and contact a personal injury attorney with experience handling sexual assault related civil damages cases without delay.
Your Daughter has a civil battery claim for money damages against the Classmate who sexually assaulted her. Pursuing the battery claim against the Classmate alone might not make economic sense, since he may not presently have sufficient collectible personal assets/income to pay any money judgment obtained. However, Wisconsin law makes custodial parents of a minor financially responsible for up to $5,000 worth of personal injury damages attributable to the malicious, wanton, or wilful act of their minor child. See Section 895.035, Wisconsin Statutes. Therefore, pursuit of joint claims against the Classmate and his custodial parents might make better economic sense than a claim against the Classmate alone.
Before filing suit against the Classmate's parents, consideration should be given to whether they have any collectible assets/income stream, since any Homeowner's Insurance policy they may have very likely excludes coverage for this type of liability. Residing in a big house and/or driving fancy cars does not necessarily mean that a person has collectible assets/income sufficient to make pursuit of a money judgment against that person economically worthwhile. There may be little equity in property controlled and some assets may be exempt from collection. The existence and nature of employment may be a more reliable surface indicator of potential collectibility. Consult with a personal injury attorney with experience handling civil intentional tort claims without delay.