Hi Martin,
Have you considered the Dataform plugin with conditional access (see HERE)? Probably the filter setting for the time (available from/to) could also appear as text behind the activity so that students would know how long an activity will be accessible. Feel free to use the attached preset and adjust the field 'Conditional activity' according to your activities. You need to install the Dataform plugin and the plugin Restriction by dataform content. Documentation for the Availability_dataformcontent is HERE.
With kind regards,
Hartmut