angularjs and template table

I would like to build an html table based on a model

I want to do something like that

Student         | competence 1                        |
                | subject 1                | subject 2|
                | exam 1 | exam2 | average |          |
xxxxx yyyyyyyyy |   10   | 20    |   15    | 45       |

And here is how i'm trying to do this

table(ng-controller="ExaminationListCtrl")
  tr
    th(ng-repeat="(competence, s) in competenceToSubjectSize", colspan="{{s.length}}")
      {{competence}}
  tr
    th(ng-repeat="subject in subjects")
      {{subject.subject}}

My issue is that i can't use colspan="{{s.length}}", it seems to me that "competence" and s are only binded to the child of th elements How could i achieve this ??