Straddle the border
The walks are probably too challenging for primary groups, but the visible dyke is a great history resource, especially when combined with one of the many other venues.
In the Wye valley Tintern Abbey is close by, while little further north, the dyke runs close to Monmouth Castle.
In the northern section there is the Iron Age Beacon Ring hillfort, and Chirk Castle. Groups staying at the YHA hostel in Llangollen can combine the dyke with Chirk Castle and the impressive Pont Cysyllte aqueduct, which takes the Llangollen canal over the River Ceirog.
School groups can gain free entry to Tintern Abbey and Monmouth Castle by contacting Cadw, the Welsh equivalent of English Heritage: - www.cadw.wales.gov.ukeducationindex.html.
Chirk Castle is a National Trust property. Contact the castle direct on 01691 777701.
For secondary schools, the dyke offers a challenging expedition. One possibility would be to take the train to Welshpool or Powys, and walk the earthwork to Knighton, which also has a train station to get the group home. This might take two or three days, depending on the group's experience.
Less ambitious one-day walks begin from Knighton, Clun, Chirk, and other points on the middle section.
The Offa's Dyke centre has detailed guides for many of these circular routes. The dyke is well way-marked and nowhere is the height more than 2,000 feet.
The appropriate qualification is the basic expedition leaders' award or the walking group leaders' award. You don't need to be a mountain leader to walk the dyke in good weather.