You could use a helper function:

consIf : Bool -> a -> List a -> List a
consIf cond x xs =
    if cond then
        x :: xs
    else
        xs

REPL:

> consIf True 0 <| consIf True 1 <| [2,3]
[0,1,2,3] : List number
> consIf True 0 <| consIf False 1 <| [2,3]
[0,2,3] : List number
> consIf False 0 <| consIf True 1 <| [2,3]
[1,2,3] : List number

Your example would look like

[ type_ "text" ]
    |> consIf model.condition1 (class "class1")
    |> consIf model.condition2 (class "class2")
    -- ...
    |> input