You will need to set the "encoder" attribute on your <t:loop /> so that tapestry can serialize / deserialize the values used to poplulate the form.
http://tapestry.apache.org/5.3.1/apidocs/org/apache/tapestry5/corelib/components/Loop.html I'd also try to get rid of your mutable static data (Session) in favour of @Injecting a singleton service. It makes it much easier to test and mutable statics are just plain nasty!
