You can control how long your announcement stays in the announcement section of your classroom by manipulating the dates. The announcement tabs are set up by Daily, last 7 days, last 30 days, and then All Announcements.
If you always want your announcement to stay on the front page, you can make it Permanent. Then it will always be there until you decide to remove it. If you set up an announcement for a particular time frame, it will move to the appropriate tab.
After the last 7 days, it will move to the 30 day tab. It will always appear on the All Announcement page. You can choose the dates when you are creating the announcement in the control panel.